Roadside service stations are not, generally speaking, the most prepossessing of places. Certainly, they rarely make me want to take off all my clothes. Yet there we were – a respectable family of four – parking our car near the petrol pumps and making swiftly for the cover of a copse of pine trees to get undressed. Finland is like that; it takes you by surprise.
Rising beside us was the Vihantasalmi Bridge, which lies around 180km north of the capital, Helsinki. It's one of the largest wooden bridges in the world, an elegant span linking two shores of a gleaming lake. The highway it carries is labelled with the numeral 5 on Finland's road atlas, but really it should have a far more alluring soubriquet. It carves north-east through forests of silver birch, past the deep greens of pine trees and onwards to Finland's glittering lakeland. It's the pathway to one of Europe's most spectacular wildernesses, not a number.
Vihantasalmi's service station is one of several along the route to offer passing motorists the chance of a swim, as well as more traditional roadside amenities. So, now clad in our bathing costumes, we took the plunge from a narrow wooden jetty that jutted from a tiny beach. The water of the lake was cool rather than cold – and utterly refreshing.
Around 10 per cent of Finland's surface area is made up of lakes such as these, strung like a soggy lace doily over the landscape. The biggest concentration lies in the south, where you find yourself constantly at the threshold of two elements: earth and water.
Helsinki, too, has this liminal quality, a coastal jewel that stands shining in the summer sun on the shore of the Gulf of Finland, close to the Baltic Sea. Cities are often difficult for families to negotiate: my wife and I might crave a little culture on our travels, but our sons Jamie, aged seven, and Peter, four, have little interest in galleries and museums. Happily, Helsinki offers the ideal compromise in the shape of Suomenlinna, a pair of islands that lie a short ferry-ride from the bustling harbour.
Once the site of a fortress built by the Swedes, who called it Sveaborg, Suomenlinna now contains a pleasing hotch-potch of museums, cafés and galleries, all set among what remains of the original military buildings. To the south lies a small cove where Peter – once he'd got over his fear that Finland must, logically, be full of sharks – paddled near the sandy shore. Beyond lay a field of ruined bunkers and obsolete cannon which Jamie and I explored. Both children were particularly impressed by the submarine on display nearby, a relic of the Second World War.
Our ferry back to the mainland passed the striking red-brick walls and gilded towers of Upenski Cathedral, which guards the entrance to Katajanokka island. Here, lurking behind some impressive square-faced art nouveau buildings, lie the high walls of what was until 2002 Helsinki's prison. Two years ago the site reopened as the Katajanokka Hotel, and – much to the ghoulish delight of the boys – we stayed in its converted cell block: three galleries created with grim utility in mind, linked by steel staircases.
Architecture aside, there's nothing spartan about the Katajanokka. Not least because it seems that the modern family requires three cells knocked together to sleep in prison-chic comfort. Our room was a tasteful concoction of dark patterned wallpaper and cream-coloured walls, with a crisp stripe to the carpet, red velvet armchairs and a bathroom bursting with gleaming chrome. The windows lacked bars, the bars stocked beer, and that evening the Jailbird Restaurant (tagline: "Bread and water, yes, but also proper cake!") served us up a hearty meal of fish and cream sauce.
We hadn't come to Finland to inspect its former jails, however funky they might be. Our goal was to tackle Oravareitti, the "Squirrel Route", a 57km nature trail which runs south-east from Juva in the Finnish lakeland, to Sulkava. On the road map, it's no distance at all, easily obscured by a crumb of Finnish crispbread; you could drive between the two towns in about 40 minutes. But it would take us two days of solid endeavour, even if we really put our shoulders to it. Because the Squirrel Route is a nature trail designed for Finland's lakes: you follow it by canoe.
The start of the route is at Juva Camping, a site on the northern shores of Jukajarvi – a lake which stretches for roughly seven of those 57km. We arrived after a 300km journey from Helsinki that had grown more beautiful the longer we'd travelled, as route number 5 offered up a verdant pallet of greens under an expanse of blue sky, itself a backdrop for scudding white and grey clouds.
Scudding too fast, it seemed. Markku Salonen and Jukka Petaja, who greeted us, were concerned about the weather. A thunderstorm was imminent, said Markku, and tomorrow would be windy. Perhaps too windy for canoeing.
That seemed impossible as the evening light glittered on the still water of the lake. From the veranda of our tiny log cabin, we watched the slow descent of the sun as the boys played on a waterslide at the lake shore. The days are as long in the Finnish summer as they are short in the winter; long past the boys' bedtime it was still light enough to read by.
The threatened storm passed in the night, but the wind had indeed grown stronger. Markku suggested we skip Jukajarvi and start instead at Toivio, a sheltered river section of the route about eight kilometres downstream. How many people canoed the Squirrel Route each year, I wondered, as we drove south to our departure point. "About a thousand," said Markku. "But only about 20 British people." He paused. "The British always finish the route, though." Ah. No pressure, then.
We'd be travelling by open-topped Canadian canoe. Juva Camping supplied it, along with a grand selection of provisions selected by Jukka, plus a camping stove. We also received essentials such as mosquito repellent, life jackets, safety helmets (for the rapids), drinking water and a map. Finally, our mobiles phones were placed – with a reverence to be expected of the country that invented Nokia – into waterproof pouches that hung round our necks. We'd use them to call Markku when we got to the end of the route, or if anything went wrong.
The boys were thrilled by all the paraphernalia. But they had every reason to be cheerful. It was their parents, after all, who'd be doing all the paddling, while they sat together on a child seat in the middle of the canoe. Then Markku waved us off – and we were suddenly alone on the river.
We paddled for seven hours on that first day, passing from lake to river, to lake again. It was strenuous work, but it was also part of a real family adventure. The boys loved the gentle rapids we encountered, took it in turns to paddle with our spare oar over calmer water, and were as entranced as we were by the scenery. Small things such as the reed beds and water lilies were as fascinating as the sheer hugeness of the lakes, or the sight of a beaver's workmanlike dam clogging a nearby tributary.
The journey was punctuated by numbered information boards that served the dual purpose of describing the local flora and fauna and reassuring us that we were going in the right direction. Our map pointed out alternative versions of those Finnish roadside service stations – places to park our canoe and pause for rest and refreshment, or stay the night – each with a wooden shelter, firewood, and a compost toilet supplied. Even the rigmarole of mooring and unpacking for lunch was an exciting diversion.
But it was our isolation that really struck home. Aside from the occasional wooden summer house there was almost no sign of human intrusion into the landscape. Indeed, throughout our first day's paddling we saw no one else on either water or shore. Despite the route's name, we drew a blank on squirrels as well.
That night, we stayed at Sulkavan Oravanpesat, a campsite with a wide green lawn, set at the mouth of a river. The owners, Pasi Vironen and Kirsi Mikkonen, were incredibly welcoming, showing us round the site (Finns, Kirsi said, are particularly fond of their communal campfires and cosy wooden shelters) before suggesting to us that we take a sauna to ease our aching muscles.
Finnish saunas – ideally of the traditional wood-burning type – are as much a part of the national psyche as Alvar Aalto architecture or Tove Jansson's Finn Family Moomintroll stories. I'd never sampled one before, but Kirsi talked us through the routine, and soon the whole family was seated, stark naked and sweating, on paper towels, in a very hot hut. The children thought the entire process was hilarious, particularly when we ran down to the lake shore to jump V C into the cold water, then made our way back to reheat in the sauna. Reservations about my modesty aside, it was certainly a relaxing way to wind down. Finally, after a substantial plate of meatballs, mashed potato and creamed beetroot supplied by Kirsi, we fell asleep in a snug log cabin by the shore. Through the birch trees, the lake reflected an ever-changing sky.
Another day, another seven hours at the oars: 30km of water to negotiate this time. I won't pretend it was all fun. The mosquitoes didn't live up to their "Finnish Airforce" nickname, but proved intermittently bothersome. And the long slog down Kuhajarvi, the last lake on the route, was a battle against a spirited headwind carried out in driving rain. However, the rapids at Kuhakoski added some drama. Markku had suggested we carry the canoe round them, but the boys were having none of it. So, after a brief inspection from the safety of an adjacent bridge, we shot the white water to great whoops of excitement.
There was a real sense of elation, too, as we finally pulled in to the shore near Sulkava's pretty marina, with a triple rainbow rising in the blue sky in front of us: our very own Finnish Line. Even Markku seemed quite impressed.
"You must be in good physical condition," he said. But didn't you say all British people finished the route? "Yes, but they do it in three days!" he laughed. "They enjoy themselves!"
In retrospect, three days might well have been more relaxing. Certainly, if the children had been older and we'd been doing the route in two canoes, we'd have needed the extra time. But now, at least, we had some shore leave to savour.
The nearby town of Savonlinna dazzles. Set on a series of islands between two vast lakes, its most striking feature is a huge 15th-century fortress called Olavinlinna, which stands aloof on an island all of its own. The next day, having recovered from our exertions, we took the 45-minute guided tour of the castle, then had lunch on a terrace overlooking Savonlinna's pleasure boats as they busied back and forth. The annual opera festival was under way, and the place was busy: blond infants swam nearby as their parents chomped ice cream, presumably discussing the pros and cons of Madame Butterfly. Later, we took a stroll with our own (sandy-haired) children along Olavinkatu, the pretty main street, which is lined with painted wooden buildings and pricey shops, and occasionally opens out on to impressive views of tree-clad shores.
Beyond Savonlinna lies Saimaa, Finland's largest lake, home to the freshwater Ringed Seal. It is officially endangered, with only a few hundred of the species still alive, but it can often be spotted as it moults its winter coat on the shores at the beginning of summer. Linnansaari National Park occupies many of the scattered islands within the lake, and ferries run to the largest of these from the canal town of Oravi, or from Porosalmi, which lies to the west. It's a place for the active: kayakers, hikers, campers, and – in the winter when the lake freezes – Nordic skaters.
Still glowing from Markku's "good physical condition" line, I was now sure we all fitted in perfectly. But, as Kirsi had pointed out, you can take things easy in Finland, too. We spent our last couple of nights in a smart complex of about a dozen summer houses just outside the town of Rantasalmi. Below our cabin there was yet another lake – the beautiful Suuri Raudanvesi – for the children to paddle in, and our own jetty and rowing-boat. Naturally, there was also a sauna. Feeling like a real Finn, I soon had it fired up.
The writer flew with Blue1, a member of the SAS Group (0871 521 2772; flysas.co.uk ), from Heathrow to Helsinki. Single fares start from £69 and children travel with a 25 per cent discount. Finnair (0870 241 4411; finnair.co.uk ) and British Airways (0844 493 0787; ba.com ) fly from Heathrow, easyJet (0905 821 0905; easyjet.com ) from Gatwick. To reduce the impact on the environment, you can buy an "offset" through Abta's Reduce my Footprint initiative (020-3117 0500; www.reducemyfootprint.travel ).
Hotel Katajanokka, Vyokatu 1, Helsinki (00 358 9 686 450; bwkatajanokka.fi ). The Finland Family Special package costs €99 and includes accommodation in a Premier Room for two adults and up to two children (sharing a sofa bed) and breakfast.
Juva Camping, Hotellitie 68, Juva (00 358 15 451 930; juvacamping.com ). Cottages start from €200 per week.
Sulkavan Oravanpesat, Kalajarventie 16, Sulkava (00 358 400 938 076; oravanpesat.fi ). Cottages start from €110 per night (sleeping up to 10 people).
HoviHoliday Cottages, Multamaentie 12, Juva (near Rantasalmi) can be booked via Rantasalmi Travel (00 358 504 310 525; rantasalmitravel.com ). Cottages start from €120 per night or €650 for seven nights.
Most visitors intending to tackle the Squirrel Route start at Juva Camping (see above), which rents Canadian Canoes for €25 per day and kayaks for €17 per day. The campsite also offers drop-offs or pick-ups along the route. Equipment (including child seats) and supplies for the journey are charged extra.