Lake Chabot - A nice quiet trail which is hidden in the crowded East Bay hills right outside of Castro Valley. Nice views, some nice hills but no (legal) singletrack and no technical trails
Lake Del Valle - Near Pleasanton
Los Vaqueros Watershed - About 10 minutes west of Discovery Bay off of Marsh Creek Road there are over 30 miles of trails at Los Vaqueros Reservoir.
Pleasanton Ridge - A multi use trail which follows along the ridge line west of the 680 Freeway. Lots of climbing and many miles of trails with great views.
Round Valley Regional Preserve - 15 minutes west of Discovery Bay off of Marsh Creek Road. Round Valley Regional Preserve is a small trail system with about 6 miles of trails (for a 12 mile out and back). There are also connectors to the Los Vaqueros and to the Morgan Territory Regional Preserve.