Tim is right, this place is a workaholic pressure cooker and sometimes you just veg on a weekend. But I've seen informal bunches of film users on weekends on Treasure Island by the main gate of the old Navy base and at the rest area on 101 just north of the Golden Gate. All looking for the definitive bridge shot!
I'm just a couple miles south of the foot of the bay. Out of the industrial ugliness there are some neat spots around Alviso and the baylands. You can hit them before work or on a lunch hour- though the noon light is a challenge. I shoot MF and 35mm, B&W with a little E6.