Be ready for very unpredictable weather, especially in the city of San Francisco, where different neighborhoods have different climates thanks to the very steep hills. When the fog rolls in, it'll fill up a gven urban valley while the sun is out just over the next hill.

Also, because of the fog effect and the relatively high moisture experienced during summer months, July and August see some very cool weather in the City and other areas west of the bay. You'll need a sweater or jacket some evenings.

I remember reading that October is actually the warmest month of the year in San Francisco(!), since that's when there is the least fog and the sun is always out. I've never been there in October, so I can't verify this by my own experience, but I did spend a couple of summers there years ago, and was amazed at how chilly some July and August evenings could be.

East of the bay, in Oakland, Berkeley, etc., the weather is completely different ~ summers are much more "normal," sunny and wearm. North of the Golden Gate, in Marin County, the coastal towns (Stinson Beach, Bolinas, up to Point Reyes) have the same kind of intermittently cool foggy weather as does the City, but the more populated areas east of Mt. Tamalpais experience a regular sunny summer.