As far as playing an actual game, I'm not sure what to do. If you go into Berkeley I think there's open gaming at Games of Berkeley near the campus.
If you are looking for gaming inspiration:
Go visit Fort Point underneath the Golden Gate Bridge. It's a real honest-to-God fortress. Buy a map and use it in your game!
Right across the bridge is the Marin headlands, overlooking the Pacific. There's many small forts up there, and it makes a really neat hike. There is also an old Nike nuclear missile station up there that is open to the public once a month, but I'm not sure when.
No matter what you do, get across the bay into Berkeley and go to Zachary's Chicago Pizza. I cannot stress this enough. I take every out-of-towner here and it never fails to impress. This is no greasy pepperoni joint. Zachary's is to pizza what Michelangelo is to that guy on TV who paints fluffly white clouds. It is the divine manna of God Himself.