Time to go back and be a child again. I spent a good time inside the Exploratorium doing some scientific experiences, a place where you can explore your 5 senses in a variety of science, art, and human perception experiments.
Be sure to have at least 3 hours to spend there, the place can get crowded really quickly and there are lots of interesting stuff to try.
My favorites were: Batman and diving penguins, the 3D mirror, the Dancing Drops and the Sip of Conflict, a really weird but very awesome way to drink water through a toilet.
After exiting the museum, don't forget to enjoy the beautiful view of the city by the piers. There are great restaurants nearby and Coit Tower gets lit at night, turning into a very charming landscape.