If you're going to pick just one walk to do in the North Island of New Zealand, make sure it's the Tongariro Northern Circuit. Mountains, rainforests and river valleys are reasonably common environments all over the world, but active volcanoes are much less common and Tongariro National Park boasts a seriously impressive collection.
The walk starts off with one of the most amazing day walks on the planet; the first day of the Circuit shares its path with the famous one-day Tongariro Crossing, which takes you right through the still-active volcanoes of Tongariro and Ngauruhoe. Both of these are simply stunning and they pump out impressive amounts of sulphurous gases and weird and wonderful chemicals; as a result the whole area is dotted with beautiful turquoise lakes with yellow crusted edges that look like something out of The Lord of the Rings. Add in the deep red hues of the Red Crater and the immense black lava flows coming off Tongariro, and you're in for a real treat.
It doesn't stop with the Tongariro Crossing, because the second and third days of the Circuit take you through some of the most desolate moonscapes on the planet and in front of the huge slopes of Ruapehu. In the summer the colours of Ruapehu's slopes have to be seen to be believed, and it's hard not to be impressed with the sheer power of the place.
There are bigger volcanoes that you can trek round – Gunung Rinjani is a good example – but Tongariro is both accessible and world class. The only thing that can spoil your trip is the weather, but it's worth waiting for the clouds to clear to see the sights of Tongariro, Ngauruhoe and Ruapehu.