If you MD in the SF parks, beware of used druggie needles. School grounds in the Mission District might be good to try. The illegal alien kids who attend those schools love to wear gold jewelry. Good luck, and let us know what happens.
Try Stenson Beach which is on Coastal Route 1 accessible just north and westerly of the Golden Gate Bridge. Fun drive if you like squirrely roads. Its a former hippy haven and now a popular surfer beach.
San Francisco itself is a asphalt / concrete city with very scarce open-spaces...and the parks and beaches may have restrictions.
Golden Gate Park is OK to hunt, Stinson and Ocean beach are way off limits. If you find a demo in SF hunt the hell out of it. The two comments above, although funny are not true, Golden Gate Park is HUGE, try Big Recreation, 7th and Lincoln, the panhandle of GGP is good for loads of change, any city or state park is good to go, national and federal land os a no-no, historical areas or off limits as well, like the Presideo, bummer. If you want old, like Reale old, try South of San Jose, Monterey area, but that would require some pretty serious research. I have had good luck recently at some demos in the East Bay... You just never know.