Bottle Lake is a great place to find varied and fun mountain biking. You can spend a couple of hours going around with the kids, or go hard for a 45 minute workout.
The most popular starting point is the main car park off Waitikiri Drive. You have a couple of options once you head up 22nd Avenue. The track that veers off to the right follows through the forest on an easy, reasonably flat track out towards the beach. A 4WD track follows the coast through the sand dunes. You can carry on all the way up to Spencer Park, or turn left after the land fill to head back into the forest for a more varied ride, meeting the coast track near Spencer Park.
The other option is to head into the forest on your left from 22nd Avenue. The track is more up and down with tighter corners. This track eventually heads up to meet the coast track near Spencer Park, where you carry on back to the main car park.
There are other starting points from the end of Bower Avenue or Spencer Park.