Just took my 7 and 10 year old who can both be tough to please (they've been all over the world at this point) to SF and Napa. I was very pleasantly surprised by the California Academy of Sciences-- it had enough variety that both kids found things that interested them and were at the appropriate level for them; the facility was nice and well organized as well. Also, if you start to feel overwhelmed by crowds and the city, the Lawrence Hall of Science in Berkeley was very fun for the kids. Its small and out of the way, but there were no crowds and lots of hands-on exhibits, so they could play (and hopefully learn) without much effort (and I got to go to the Cheese Board in Berkeley after).