Inside the airport there are several expositions. If you attempt to go to a famous place like the fisherman's wharf, you will spend at least two hours commuting.
Seems like a lot of time to kill but I would advise against going off site. What with security returning to the airport and travel time it doesn't give you all that much time. Staying at the airport is less stressful. Just do a Google search for " activities at the San Francisco airport"
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