I just heard that a couple of local museums are participating in
Free Museum Day - California Academy of Sciences (temporarily downtown), the Exploratorium (in the Marina), & the Museum of Craft and Folk Art (also downtown).
I don't think I've visited the Exploratorium, having confused it with the Academy for ages. So I'm heading thataway.
EDIT: Oops! You have to
print and bring this card / coupon. Sorry about that.