Go see a Giants game if you can but if you’re new to baseball, try to find someone to explain what’s going on. I took a colleague there and despite his protests that it was “only rounders”, by the end of the game he was hooked and bought tickets for two more games while we were there.
It’s a really walkable city too so don’t feel obliged to use public transport. I walked from downtown, through chinatown and back to my hotel near fisherman’s wharf.
Go down to the Mission district for burritos and bikes or if you can, rent a bike and see it all from the saddle.