I'm in Mountain View for a couple of weeks and since it happens to be also labour day I asked people around on what to do during the week-end. I put the summary here so that other visitors in the San Francisco area can take a look at it.
- Bike the Golden Gate Bridge (It's about 15-20 km and may take a couple of hours). You can rent a bike in San Francisco, bike to Sausalito or all the way to Tiburon, spend some time there and then take the ferry back to SF
- Bike or walk along the waterfront in San Francisco is nice. You may even combine this with the Golden Gate Bridge bike ride
- Bike (but beware of the many annoying hills and cars everywhere!)
- You can take a bike on CalTrain to San Francisco. The bike car is the last car on the train.
- Bike on a road bike along the east bay with http://www.oaklandyellowjackets.org/
- Visit Alcatraz (tickets must be reserved in advance)
- Walk around in San Francisco with a guided tour:
- Walk around the Mission district (Valencia st. between 24th and 16th, 24th between Valencia and Potrero) is fun. Check out Paxton's Gate shop, bookstores, restaurants, murals, etc. Humphrey Slocombe ice cream is good
- Visit Angel Island, especially the newly renovated Immigration Station
- If you enjoy hiking and are in decent shape, I think a Half Dome hike is an incredible experience: http://www.nps.gov/yose/planyourvisit/halfdome.htm Unfortunately, it's a long drive (3.5+ hours from SF)
- SF Giants baseball game on Monday
- Go to a museum in San Francisco
- Go to a museum in Mountain View
- Drive around Silicon Valley.
- Drive down Highway 1 toward Hearst mansion / Los Angeles.
- 2 days is enough time to take a pretty leisurely trip to the sequoia national park
- Rent one of those yellow buggy cars and drive around SF
- Check out Big Basin state park - the redwoods there are amazing.
- The aquarium in Monterrey is worth a visit too.
- If you have more time, go to Yosemite and the over Tioga to Mono Lake and Bodie, sleep in Lee Vining (just next to Mono Lake) as other places maybe full already. The places are spectacular!