Hmmm...here's my short list:
Take the trolley cars (there are 2 lines) end to end. One goes through China Town (great places to eat--I like the Empress), the other goes to Fishermen's Wharf. Interesting things to see at each end of both lines.
Walk up to Pier 39 from the Wharf (5-10 minutes). Very touristy outdoor shopping, but great fresh Dungeness (sp?) crab right now!! Cooked while you wait. Sample crab cocktails and other fun, cheap seafood dishes at both the Wharf and Pier 39. Avoid the stores offering cheap pearls you can dig out of your own oyster--they'll talk you into a $500 setting.
Stop and gawk at all of the sea lions barking at each other on the north side of Pier 39 (behind all of the stores). Better than any zoo I've seen.
Take the "F" streetcars in a loop from Market Street around past the Wharf (again!?). They are all 1930's to 1950's resurrected streetcars from all over the world. Again, very touristy, but cheap and interesting. Also provides an easy way to get up and down Market Street and the Embarcadero (which is where all of the piers and the Wharf live).
Take the BART (subway system) for quick and easy movement in and around the city, especially at rush hour, since the streets jam up. It doesn't go everywhere, but it covers quite a bit of territory, especially connections outside the city.
Go up to Coit Tower. Great view of the city, cheap access, interesting historical landmark, but you need a car to get there--maybe a taxi. I've walked it from the Embarcadero--about 20 minutes uphill, though.
Walk through China Town. Lots of interesting things and people to see. Mostly stores and restaurants. Great food, as I said before.
Drive over the Golden Gate Bridge, then walk it as well. Very good view spots on the north side in Sausalito. While you're there, take some time to go into Sausalito, down by the water. Very artistic and laid back area.
Buy tickets in advance for the boat trip and tour of Alcatraz island and prison. Cool (literally--bring a warm coat).
Go down to Cupertino (45 minute drive) and have your picture taken in front of 1 Infinite Loop, then go see the Apple campus store--not big, but eventually everybody goes in there. Even Steve. Has Apple logo clothing and lunch boxes not available everywhere else.
There's lots more to see/do, but I gotta go. Don't forget that it will almost certainly be rainy and cool most of the time. However, I've also sat outside an "In n' Out Burger" (sp?) there on Christmas eve in my shirtsleves enjoying the sun, but your mileage may vary.
Cheers,
WCat