One must go to San Francisco Fisherman's Wharf, even though we know it is a tourist trap and yadda dada. But go for a walk down the piers near sundown - it makes a lovely way to relax after a long day.
I would not recommend eating at any of the seafood restaurants at Pier 39, because you pay a lot for gimmick and not so much for good food. Walk further down to a more casual Boudin Bakery, famous for their sourdough bread that is in itself ubiquitously SF. I usually get a clam chowder bread bowl and that fills me up nicely. Or, if that is not your thing, the only In-and-Out burger is also at the Wharf.