I'll second Leila's excellent list and I'll add a couple more things.
If you get tired of SF's ultra-trendy restaurants and want something a
little more traditional go to Swan's Oyster Depot for oysters, shrimp
cocktails, etc. Don't miss SF's oldest eatery, the Tadich Grill for
charcoal-broiled fish and old fashioned ambience (and that goes for
Sam's Grill). For steaks with an Italian-American accent, go to
Alfred's. If you enjoy modern art, the SF Museum of Modern Art is a
must. You should also experience the newly rehabbed Ferry Bldg., now a
food court and then ride the J line vintage street cars to the Maritme
Museum at Aquatic Park. You can grab a cable back to downtown from
there. I f you have time , take a ferry to Sausalito, Angel Island, or
Oakland. The Oakland Museum is worth the trip. And then there's the
New Deal era murals in Coit Tower on Telegraph Hill along with it's
great views. And much, much more..

Enjoy-
D.M.