The middle of the north island is a geothermal mecca. It's got several volcanoes and some major geothermal hot spots. The geothermal activity in Rotorua is at the surface in much of the town- which means every hotel has natural thermal hot pools! We visited two geothermal parks and took fly fishing lessons.
The best golf course in New Zealand is located in Taupo-and although we did go golfing, it wasn't there (because much to Brad's dismay it was under repair for the whole week of our trip!) Taupo is a base for hiking the Tongariro Crossing- a day walk which traverses between the volcanoes Mount Tongariro and Mount Nguaruhoe (which is actually a vent of Mt Tongariro- and is also known as Mount Doom- in the Lord of the Rings). Many of the scenes in the Lord of the Rings trilogy were filmed in Tongariro National park. None of the volcanoes are very active at the moment (although Mt Ruapehu- also part of Taupo's volcano zone- is one of the most active volcanoes in the world and erupted in 1996 and 2007- and it has a ski field on it!) We did a 3 day hike through the park (one of New Zealand's great walks- the Tongariro Northern Circuit, which includes the Crossing) and although the weather wasn't the best for a couple of the days, the hike was quite spectacular.
After that we headed to Waitomo caves and went "black water rafting" (actually it's more like floating). The caves are full of glow worms, and are very long with rivers running through them. Waitomo means "water passing through a hole" in Maori.