Alcatraz.. I consider that a must see unless of course you have a record and don't really like being back in a prison :)
Other popular things are taking a tour of the city on a segway.. and these:
The Urban Safari
Golden Gate Bridge
Ferry Building Marketplace
Hope this helps! about 10 years ago
Def see Alcatraz! But BOOK Early!!!! The tours sell out fast and I would suggest even booking your ferry ticket before you arrive.
If you stay at Fisherman's Wharf you will have access to the best parts. Hang out at Pier 39 (visit the local sea lions who moved in after the '89 earthquakes and refuse to leave) and take a stroll along the Embacadero there are plenty of shops to ckeck out.
Heasd back towards Fisherman's wharf area to check out the local seafood (they are well known for clam chowder and "Chippino").
If you're a chocolate fan check out Ghiradelli Square, or the Boudin bakery for fans of sour dough.
To see most of San Fran in a day I highly suggest hiring a "go car". check out www.gocartours.com for details. They are 1-2 person gps guided cars that will take you everywhere you want to go, the best part, you are your own tour guide (with the help of the gps lady, who also gives a history of the areas you are driving through) so you get to stop off wherever you like and spend as much time at each attraction as you please.
Other sights I can reccommend not previously listed
Bay bridge and Haight Ashbury .
Enjoy about 10 years ago