Belmont regional park, is a pretty good place to Mountainbike. Its big, offers great views, with a wide variety of terrain, and lets you pretty much go from the Hutt Valley all the way to Paremata if you know where to go.
On Hill road in Belmont (very close to the action) there are a bunch of Cottages you can stay at as well, if you book in advance. If you need a cheap rental car in Lower Hutt, try out Matt and Vicki. Their rates are very cheap and they will get you a bike rack if you need it. Cheaper long term car hires in Lower Hutt are a specialty with them, and they specialise with the Backpacker crowd.
The best online bike shop in Lower Hutt is Bicycle, and they are also open to street traffic (during the week only) with very friendly hours. Closer to Belmont though is VIC Cycles, about 8 minutes ride away from Belmont.
Cheap rental cars in Lower Hutt
Out of the rat-race accomodation in Belmont Lower Hutt
Online bicycle purchases and info