Welcome-I was just in San Fran 2 weeks ago! Did the drive south down to Carmel, it was beautiful!
St. George's is a good place to start. The gymboree and kindermusik are good, but a bit pricey, but so is San Fran so it may be what your used to.
St. George's is a good place to start as they don't allow your maids/helpers to bring the kids to their playgroup, which a lot of other activities tend to get overrun by maids and the mom's stop coming.
I have a 2 1/2 year old boy and he loved St. Georges. Good luck!