Lurkette, San Francisco can not be seen within day
That said a lot of suggestions above have merit.
Personally I think Pier 39 and the Golden Gate are a must. Lombard street is also very nice to have seen once.
(actually just driving around in Frisco makes you feel like you're in a movie
with all the height differences)
Its been 6 years since I was last in SF, so there are probably many more places I just can't remember.