I think there was already a thread on this, but I hope you like climbing.
Search for these climbs: Old La Honda, Montebello, Redwood Gulch, Kings Mountain Road, Tunitas, Hwy 9, Mt. Hamilton, Mt. Diablo, Bohlman-On-Orbit-Bohlman,Sierra, Quimby.
Here's a fav of mine:
http://www.paloaltobicycles.com/resources/ring-of-fire/
Pretty much any ride that goes out the coast and back over skyline would be good.
For a quick flat ride, the Portola loop around Portola Valley near Stanford is nice.
For Mountain biking: Soquel Demonstration Forest and Skeggs for hardcore rides. Any one of the open space preserves for easier rides(These are rideable on a cross bike).
One cool, thing I plan to do this year is the twilight ride at Laguna Seca, where they let you ride around the track.