Family entertainment, ancient and modern, is on hand near Cambrils holiday park on Spain's Costa Dorada, says Natalie Holmes

It was both a question and an accusation: "How did I get to be 12 years old without having been to a theme park?"

But such a callous infringement of the rights of the 21st-century child was about to be made good, for our summer holiday was going to include a trip to the biggest theme park in Spain, PortAventura on the Costa Dorada.

This is no Mickey Mouse theme park. Its 115 hectares encompass the whole world, or at least the world reduced to five "themes": Mexico, the Mediterranean, Polynesia, China and the Wild West. Really, what more of the world do you need to see?

But first we – James, his 15-year-old sister Ruby and I – had to get there. Having sworn off flying until climatic catastrophe is officially averted, we were going by train, a surprisingly easy and even fun trip.

Our destination was the Cambrils Park, just south of Tarragona, with Canvas Holidays, which throws in a family day pass to the theme park with a week's stay. So it was Eurostar to Paris, a few stops on the Metro, then on to the Elipsos TrenHotel, which whisks you from Paris to Barcelona while you sleep (hopefully) between crisp cotton sheets in a snug little bunk. Complimentary toothbrush aside, it is more train than hotel – the bar was down to its last bocadillo about an hour into the journey – but kids (mine anyway) love sleeping on a train, making the journey part of the holiday.

There are trains almost hourly from Barcelona to Cambrils or its neighbouring resort Salou, and we passed PortAventura near the end of the 90-minute trip, the unmistakable silhouette of its eight-loop landmark ride, Dragon Khan, looking like something designed by Santiago Calatrava in the grip of a bad migraine. Excitement mounting, a taxi whisked us for the last leg to the bright green oasis of Cambrils Park – one of those enormous holiday parks that the Europeans do so well, with two pool complexes and room for caravanning, camping and bungalow living.

We were staying in one of Canvas's new Aloha (Polynesia again!) bungalows. Inside, the thatched structure looked more Swedish than Hawaiian – it was wood panelled throughout like a sauna, though the air-con ended the resemblance. Our spacious two-bedroomed bungalow was comfy for three. The kitchen was well equipped with microwave, fridge-freezer, TV, kettle, etc, but there was nothing else, no washing-up liquid, bin liners, or soap. It wasn't so much self-catering as camping in a bungalow (bamping?).

Luckily, the site has a well-stocked shop, as well as tennis courts, mini-golf, games area, and evening entertainment. Everything, in short, to make you never feel the need to leave, but the thumping PortAventura jingle had entered James's brain during his many visits to its website, and its siren song could be resisted no longer.

The start was not promising: it was too, too hot; the wait for the first big ride took an hour and we gave up and left the Mediterranean village at the park's entrance and took a slow boat to China, which was all dragons, tea-pots and sampans.

Universal Studios was involved in the conception and building of PortAventura, and the zones feel like film sets, designed with loving attention to detail if a rather broad brush – and it's probably best to leave any notions of political correctness at the turnstile and go along for the ride(s).

Still, we got into the swing of it, being twirled around in a tea cup, lashed about by a snake. After China, it was the Aztec ruins of Mexico, complete with matching vegetation and jungle sounds. Then a quick tour of Polynesia (we got enough of that at our holiday home) and on to our favourite area, the Wild West.

The town of Penitence (population 264 and falling) had a train, a jail, complete with scaffold (you could take your photo being "hanged"), graveyard, coffin wagon, school, various saloons, shops, a dentist (tooth-pulling five cents, anaesthetic $5) and a funfair in the town square. Not to mention a few rides. By late afternoon, the queues had fallen away, as had the heat, and as dusk fell, it was hard to resist the pleas for just one more hurtle down the Grand Canyon rapids.

After "the best day ever", the campsite routine of pool, lunch, siesta and ping pong took effort to resist, but there is another unmissable attraction on this underexplored coast. Tarragona, an hour away by bus, was once the Roman city of Tarraco and has the largest collection of Roman ruins west of Italy.

The cathedral, which is also a museum, provided a welcome escape from the heat, but the most stunning site was the Roman amphitheatre, set against the sea. Sections of the seats are still intact and you can wander around the arena where gladiators fought, wild beasts were slaughtered and Christians martyred.

That was enough to build a healthy appetite for lunch. And on Tarragona's main streets – the Ramblas – we found plenty of places to eat among the Modernist buildings. But the similarity with Barcelona ends there, there are no crowds, the pace is slow, and the city is small enough to explore on foot.

On the way back to the bus we passed a life-size statue of one of Tarragona's famous human towers. These "castells" are made up of people standing on each others' shoulders in decreasing circles, up to seven storeys high. I suppose that's how thrill-seekers entertained themselves around here before the theme park.

Compact Facts

How to get there

A week at Cambrils Park on the Costa Dorada, Spain, costs £1,294 for two adults and two children this summer with Canvas Holidays (0845 268 0857; Return rail fares from London to Barcelona start at £204 through Rail Europe (0844 848 4070;