We went to Monkey World in August 2010. It's a few acres of forest, that the owner has rigged up with rope bridges, swings, zip wires etc.
You start off by getting a harness and some instruction on how to keep yourself clipped to the safety wires at all times & then you're free to explore.
There are (I think) a dozen different routes through the trees, each colour coded according to difficulty. My 6 & 8 year olds loved it, and loved working their way through the easier courses, scampering up nets and through barrels up in the trees and then whizzing down the zip wires on pulleys. It really is fun for all ages and there are a few black runs which are up in the tree tops and are quite scary.
We spent an hour there but you could easily spend half a day I think. There's a cafe on site and the instructors are friendly & helpful.