For mountain bike riders, here are a couple of useful links by Charles Blackford in 7X7 magazine. It has detailed maps and pictures of the Fremont Older Open Space Preserve and the second link has a number of other great mountain bike rides in the Santa Clara County Region. Safe rides!
http://www.7×7.com/east-bay/spend-your-lunch-hour-outside-quick-mountain-bike-excursion
http://www.7×7.com/user/232