Be safe, I live in Manteca, CA (about 2hrs form there). I'll be out of town from May 10-15 so I won't be able to come visit you, but its fairly fun!
Just be careful if you are alone at night or anything like that. There's a LOT of homeless around the subway (BART).
If you have time, check out the piers, pier 39 specifically. Also check out all the street performers around there! A LOT of fun!