There are lots of bars and restauarants around SFO, so go out and have some fun! Every once in a while I even see uniformed flight attendants in nearby grocery stores (stocking up for long int'l flights?), so a quick jaunt out to nearby areas is very feasible.
I've never transited SFO as it's my own airport. Will you go through customs without claiming and rechecking baggage? If so, you could probably get back to the airport an hour before departure and you'll be fine, since you'll already have a boarding pass. If re-checking bags, I'd personally allow a little longer, but that's just me.