This is really the hub or starting point for most MTBers in the area. There are a ton of well-marked trails along Forbidden Drive. My best advice is that if you want a good ride, take any one of the side trails.
How to get there:
From Rt 76: Take the City Line Ave exit and as soon as you get on City Line, make a u-turn. Follow City Line Ave across the bridge over the river. Take Ridge Ave exit and follow Ridge all the way until you get to Roxborough Ave. Make a right turn on Roxborough and follow it until you hit Henry Ave. Make a left turn on Henry and drive until you see the sign for Valley Green. Make a right turn on the road into the forest. Drive until you reach the parking lot.