North of San Francisco near Mill Valley, the 6,300 acre Mount Tamalpais State Park has trails winding through redwood groves and oak woodlands. The most fire roads, including the Eldridge Grade from the summit, are open to mountain biking. Make sure not to ride any trail marked as closed to mountain bikes. Also, watch your speed and limit it to the maximum posted speed (radar enforced -- no kidding!).
Tip - After a long mountain bike ride at Mount Tamalpais State Park, make sure to head over to the Gestalt Haus in Fairfax for a cold drink!
Overall, Mount Tamalpais State Park is the 111th most popular mountain bike trail of all 132 mountain biking rides in California.