Biking Callahan State Forest
Callahan State Forest in Framingham, Mass is one of my favorite places to bike, mostly because it's relatively easy (so I don't need any gear besides my bike, helmet and gloves) and really close (so I can go all the time).
Callahan has two starting points - by the golf course, and by the apartment buildings. I've done both, and we usually start by the golf course because it has that nice flat "warm up area" and the cool pond to hang out at when you're done. Plus it's by Rasoi which has excellent Indian food for dinner afterwards.
We usually do two 45-minute loops - go left on the flat part, down the steep cut near the end of the flat part, and then left at the first intersection in the woods and loop from there. It's hard to get lost in there (although I've done it once, climbing fences with a bike after dark isn't much fun). Most New England books have this one mapped out so you can get yourself a map if you're worried about that.
South Callahan in spring (May-June) is tough - it gets a bit soggy and very mosquitoey. Bring bug repellant and avoid trashing wet trails!!
On the other hand, North Callahan in late summer and fall - what gorgeous trails! Hardly muddy at all, very few mosquitoes, and great riding.
"Mountain Biking Near Boston"
an excellent reference with maps!
Biking in New England