oh man....where to start?! One of the greatest cities in the world! (at least in my opinion)
It's too bad you don't have a car. One of my favorite past times in SF is to just drive around the different neighborhoods. The architecture is fascinating.
Go to Sausalito, if you can catch a bus. If you like Sushi, I can recommend a wonderful little restaurant there:
Sushi Ran
Places to walk around:
Union Square
China Town
Palace of Fine Arts
Lombard Street - any way you do it, you've got to see it!
The Haight-Ashbury
MOMA
Telegraph Hill
Colt Tower
Make sure you ride the Cable Cars!
On Saturdays, there's a wonderful wonderful wonderful (did I say wonderful?) Farmer's Market at the Ferry Building Marketplace. You can get great food there and sometimes there's some pretty famous chefs cooking up a storm!
And, if you're going to be driving down to Monterey Bay, then stop in Carmel! It's a beautiful, charming little town!
Have a great time. I LOVE SF. Wish I could move there. You can't go wrong with restaurants...the selection is fantastic!