The thermometer shows minus 8 degrees Celsius. Our steps have left deep imprints in the snow and our breath leaves clouds of white in the air. But my 5-year-old is undeterred.

Dressed only in a swimming cap and bathing suit, she plunges into the water. "Look, mum," her 9-year-old sister calls. "Now we're even having a beach holiday."

We are in Iceland, and it's the middle of winter. We're trying out the Myvatn Nature Baths, with their waters of around 40 degrees, the smaller sibling of Iceland's famous Blue Lagoon spa, which is at the other end of the country.

The countryside around the Myvatn Lake is particularly full of variety. Bizarre lava formations topped with snow surround the lake like turrets. Waterfalls plummet into the depths and icicles sparkle. In hollows and crevices, boiling water bubbles.

"All of volcanic origin," says Halldor Ingvason. My eldest daughter listens attentively to the tour guide's every word, while her younger sister is eyeing up the the vehicles that he and his colleague have parked at the side of the street: black snowmobiles. 

They are each three metres long and weigh around 300 kilos, and they are our means of transport around the lake. "The coolest way to get from A to B in winter," he says.

We take off, passing by sheep that allow themselves to be stroked and island horses that can winter outside courtesy of their thick coats.

Finally, we go on a whale and sea fishing trip, setting off from the fishing village of Hauganes. 

While my eldest keeps a look out for humpback whales and dolphins, the youngest casts her rod into the water and promptly snags an arm-long cod, which we later eat for dinner.

We also visit the ski resort of Hlidarfjall, whose 14.9 kilometres of slopes are very manageable, though experienced skiers can ski from the mountain down to the ocean in many places.

In the evening, as we lie in the hot tub on the terrace of our accommodation, my youngest points at the sky and shouts, "coloured clouds!"

"The Northern Lights," my husband says. I briefly think about jumping out to get my camera but then think better of it, and lie back to enjoy the show.