Home of the Mountain Bike
One of the best things I've done in San Francisco is Mountain Biking in Marin County. I imagine you could easily organise a similar trip here yourself but if you're new in town, bike-less and not sure on the trails mountainbikesf.com do an ace job. For $175 Mark at MountainBikeSF organised a great half day trip across a few different trails packing the session with technical lessons, history and fun times.
All up we were on the pedals for 2.5 hours covering off just under 27km with 730m in elevation. The day incorporated a few climbs early on, some great scenic flats as well as rolling switch backs and descents (the highlight).