Should you ever find yourself in the central area of New Zealand's North Island, then the Tongariro National park should be somewhere on the top of your list, if not number one.
This compound volcano is located some 20 kilometers southwest of lake Taupo, and together with his siblings - Ruapehu and Ngauruhoe - they form the still active volcanic zone that dominates the landscape of the island. The last eruption at the time of this writing was on the 6th of August 2012.
Maori legends say that, long ago, seven mountains stood close together around Lake Taupo, all being male except Pihanga. So naturally, one day the men started fighting for her favour, spitting smoke and fire while shaking the earth. In the morning Tongariro stood victorious next to Pihanga , while Ngauruhoe and Ruapehu stood a respectable distance behind. Taranaki even fled all the way to the shore of the Tasman Sea.
The Tongariro National Park was New Zealand's first national park as well as one of the earliest in the world. If you have time to spare, the Tongariro Northern Circuit is one of New Zealand's "5 Great Walks" and is a stunning 5 day hike around and through the volcanic massif.
A shorter alternative would be the Tongariro Alpine Crossing, which is an equally breathtaking 1 day hike between Tongariro and Ngauruhoe up to the famous green emerald lakes (the name is well deserved). Just be aware that although this is a relatively easy hike, it is still some 20km long and quite steep in places.
The national park itself is free (like all national parks in New Zealand), however you will probably have to pay for accomodation the night before (you want to start early) and for the shuttle that brings you to the start of the track and picks you up on the other side (you can bypass the shuttle if you have two cars at your disposal).
Be aware that the hike is very exposed and strong gales are no rarity. Also, in case of bad weather, the access to the track might be closed.
Oh, and have a good look at Ngauruhoe on your way up and hope he doesn't errupt - it's Mount Doom from L.O.T.R.