I did a lot of fun stuff last week when my college roommate was in town. Probably the most photogenic was going to see the Chihuly exhibition at the de Young museum in San Francisco.
More of my photos here.
I'd definitely recommend going on a weekday if possible. Although we didn't have any trouble getting tickets when we got there, it was rather crowded. And skip the cafe, no matter how good it smells. Unless you want to pay $7 (and wait 30 minutes) for a hot dog just for the experience.
We also went to the San Jose Museum of Art, which was actually really cool. There's a lot of wacky hi-tech installations, and there's a robot theme going on now. I think it would be really fun for kids. There's a table where you can build a Lego robot & have it photographed and uploaded to Flickr. My friends' are in there somewhere.
My friends went to the Tech as well and really liked it. They came back with a robot-drawn portrait of themselves. Admission is only $8 and includes an IMAX ticket.