There are more than a weekend's worth of activities in the San Francisco Bay Area!  To help get you started, here are some of the most popular sites to visit:
- The Golden Gate Bridge
- Walking along the Piers
- Alcatraz (reserve tickets at least a few weeks before coming)
- Lombard Street in San Francisco (The famous crooked street)
- Vermont Street in San Francisco (The real 'most crooked' street in the city)
- Wine Country (Sonoma and Napa - about a hour's drive north)
- Driving Highway 1 along the Pacific Ocean

Here are a couple helpful links:
Yelp San Francisco (Reviews on just about anything in S.F.)
SfGate Visitors Guide
Bay Area Critic - Things to do in the Bay Area
Mapwest Top 20 for S.F.
Yahoo Trips
S.F. Station (Clubs, Music, Art)
Museums in S.F.