Check out Zeum this Saturday- it's free admission day! This San Francisco's Children's Museum, in Yerba Buena Gardens is a fabulous hands-on place. Hightlights include an interactive claymation studio, music production lab, Lil Z's Play Lab (for tots), and exhibit galleries.
While you're there, you can ride the historic Carousel on 4th St. and Howard (I think it's $2/ride) and play in the free Yerba Buena outdoor playground. Plenty of restaurants nearby for lunch too.