The Craters Mountain Bike Park has about 50km (30 miles) of trails to suit all skill levels. They meander through pine and eucalyptus plantations.
The trails are maintained by a not-for-profit trust called Bike Taupo and you have to pay to ride the park. You can do that at the Hub Cafe or at bike shops in town. The cost is $10 for a week and you while you're at it you should pick up their really good map for another $3.
A good trail for beginners to start with is Tourist Trap. Intermediate or better riders could head up the hill on Hammer It, Tank Stand, and Grinder. Deb's Trail should not be missed, then cross the road and try Mr and Mrs and so on. On the way back to the car park don't miss Coaster which is fast, with big berms, jumps and is loads of fun. Don't bother with Outback if your time is limited.