If you find yourself sitting home during the summer weekends in San Francisco with nothing to do but watch the Giants, let me make some suggestions. These are activities that take a half day to a weekend, and can easily be approached by nearly all would be participants.

Quick and Dirty: spend an afternoon out

Indoor climbing - play spiderman.
Skydiving - not just for nuts.
Wine tasting - we live in the center of US winemaking.
Bicycling for views.
Dragonboat paddling - a couple hours a weekend for a short or long season.

Slightly more time: two half days to learn

Kayaking on the bay or coastal estuaries.
Sailing - both windsurfing and small sailboats are available all over the Bay.

Full day plus events:

White water rafting on inland rivers.
Skiing in Tahoe.
Hiking in the coastal range, or elsewhere in the state.
Caving in the Sierra Foothills.

Long weekend endeavors:

Learn to ride a motorcycle with MSF.
Learn to dive - get certified to dive in Monterey or in warmer waters.