I am going to San Francisco for three nights... maybe four... Now what? What is there to do there?

The evening we get there we are going to alcatraz. and then our last day there we are going to a concert (Rock The Bells). What should I do in that middle day? I also might come a day early if the funds get saved right. It is me and my GF. might go to rainforest cafe but heard its not worth the money. any ideas on what two love birds can do in san fran in a day or two?