Kennedy Park - Lenox, MA
Warning: Horses are allowed. Always yield to horses!
My favorite mountain bike ride is a 7.25 mile loop that is mostly singletrack. It's way too difficult to describe the loop so go buy a GPS and follow this track:
When you're done, stop in at the Arcadian Shop. Have a coffee and a snack, surf the web and buy some gear! Or just go in and talk with the people there! It's a great store.