Sinking lower into the warm water, I inhale a lungful of crisp air and gaze upwards. Night has fallen, dusting the sky with stars and dashing it with a faint, green haze of the Northern Lights. Candles illuminate my surroundings with their flickering glow.
The natural hot spring I'm sitting in, a 90-minute drive from Iceland's capital, Reykjavik, is a paddling pool-sized hole, surrounded by grassy clumps, thick snow and a smattering of trees. Relaxing with a beer in hand, it's a prime location for watching the aurora borealis - but I'd never have found it by myself. And, judging by the fact we are the only ones here, neither would other tourists, many of whom head to the country's best-known geothermal pools, such as the Blue Lagoon.
Many travellers want to get off the beaten track - but finding secret spots such as this requires local knowledge, something that Icelandair is offering in its new "Stopover Buddy" scheme. The concept is simple: transatlantic passengers stopping in Reykjavik en route to North America are paired with an airline staff member to act as their personal, free tour guide for up to 10 hours.
Choosing from six themes (lifestyle, nature, health, culture, food and adventure), you are matched with the member of staff specialising in that field, from off-piste skiing with the CEO to sea fishing with cabin crew.
You can bring up to three people, and while you pay for your own activities, the airline picks up the costs for your buddy, and pays them a day rate for their time.
My buddy is Siggi Anton, the airline's 37-year-old website manager - and a part-time mountain rescuer - who has created an adventurous itinerary for me.
Not only has he driven me to his favourite spring deep in the countryside (the airline covers the cost of transport to and from activities), he has also brought everything I would have forgotten: towels to lay on the snow while dressing, gowns and bottles of local brew.
Before meeting up, buddies can also email tips for activities for you to do solo, so I begin my two-day trip with Siggi's recommendation - a sea swim at Nautholsvík geothermal beach. Joining a handful of locals, I watch the sun rise from a 39°C hot tub - before dashing over frosty sand and launching myself into the icy sea. I last just seconds, but leave feeling invigorated and ready for the day.
Knowing I'm a keen cyclist, Siggi has suggested taking me on a 15km bike ride around the western area of Reykjavik, which, he says, is both picturesque and traffic-free. After meeting at Icelandair HQ, next to Reykjavik's domestic airport, he drives us to Harpa, the futuristic, harbourside concert hall, where our hire bikes await.
"The best way to see Iceland is by bike," Siggi says, clipping up his helmet. "You see so much more than you do by walking. And unlike in a car, you experience it through all your senses, from the way it smells to how the air feels on your face."
I soon understand what he means, rolling up my balaclava to stave off the freezing wind as we set off, beneath a cloudless winter sky. Our tyres crunch through the snow as we roll past the Grandi harbour area, dotted with fishing boats and lined with a merry cluster of turquoise buildings. "New York has the meatpacking district; this used to be our fish-packing district," Siggi says.
These buildings - once fishermen's huts used for repairing boats and nets - have been transformed into bars and restaurants. One of them, Slippbarinn, is furnished with cosy booths and comes alive at happy hour.
A line of people spills out of another harbourside building, now home to Valdis ice cream store. Even when temperatures are sub-zero like today, people queue for a taste of its signature Danish liquorice ice.
With views across the sea to snowy mountains, we push on, heading towards the lighthouse on Reykjavik's westernmost islet, Grotta. On the way, we pass two women warming their feet in a steaming hot pool over a flask of tea, and pause to peep inside a wooden shack, where hunks of shark meat, reeking of ammonia, are hanging to dry. It will later be sold at Kolaportio, Iceland's biggest flea market, alongside second-hand clothes, vacuum-packed lamb testicles and foal meat.
With the afternoon sun now low, casting pools of gold on the water, we cycle around the headland, with its frozen golf course, and back towards the city along Aegisida, the waterfront street where Björk lives. As we near the city centre, Siggi points out a grey house on the corner, home to Vigdis Finnbogadottir, Europe's first female president. On 30 June 1980, she stood on its balcony waving to the thousands of supporters gathered to congratulate her after the results came in.
I realise with a sense of satisfaction that we haven't passed another cyclist all afternoon, let alone another tourist on two wheels. Having worked up an appetite, we head for dinner at Apotek, a centrally located hotel and restaurant inside a former pharmacy. Elements of its old purpose are visible in the marble interior - there's a wall of medicine bottles, while "pharmacists" mix cocktails categorised as "painkillers", "stimulants" and alcohol-free "placebos".
The buzzing restaurant fuses Icelandic and European cuisine with an Argentinian asado. The duck and waffle starter is beautifully presented and an interesting blend of sweet caramel and meat, while the rack of lamb is juicy. Over chocolate patisseries and scoops of rose-shaped sorbet, I wonder if there's a catch to what's essentially a free tour guide service.
Currently, it's subject to the availability of buddies (there are 50 on the books) and is only running until the end of April. But Siggi has other thoughts.
"The idea is to give people a taste of Iceland," he says. "The catch is that they'll want to come back."
GETTING THERE Icelandair (icelandair.co.uk) flies from Gatwick, Heathrow, Manchester, Birmingham and Glasgow and, starting in March, Aberdeen, with onward connections to 16 North American airports. Return fares start at £379.
VISITING THERE Transatlantic passengers can stay in Iceland for up to seven nights for no additional airfare. To request a Stopover Buddy, visit icelandair.co.uk/stopover-buddy.
MORE INFORMATION inspiredbyiceland.com