Just want to second the bit about Academy of Sciences, it's a pretty awesome place, you can do most of it in a day, and they have stuff thats interesting for all ages, if you happen to be with some kids or anything.
Aquarium, rainforest enclosure, green roof, various presentations and all sorts of exhibits and stuff that change over fairly frequently.
One night of the week they have some nightclub type thing, but I've never been to it, so I can't speak about that.
Plus there is plenty of other cool stuff in Golden Gate park (where the academy of science is located). It's right next door to an art museum (you can go up in the tower for free), there's the Japanese tea gardens, the botanic gardens.
Over by fisherman's wharf, there's a hill with the coit tower on it, it's pretty steep, but sometimes you can see the parrots if you are walking around there (you can almost always hear them)
If you are going to be in for more then a couple days, there's all sorts of interesting things in the nearby area as well.