Most of the single track is closed to bikes, but that which isn't is fun. All of the fire roads are open to bikes, of which there are miles and miles to choose from. There is also a mountain bike loop trail put in by the NEMBA and there may be more such trails to follow soon.
How to get there:
Rt. 93 North of Boston. Take exit 33,34 or 35. All put you right out at the Fells. Find the nearest parking lot and pedal away!
Watch out for anti-bikers spreading lies on and off the trails.