K.C.
Don't forget about Children's Fairyland in Oakland. There is one add ission price and then the rides and shows are free. Also Happy Hollow in San Jose is supposed to be a lot of fun, although I haven't been there in years.
I was also going to recommend Tilden Park and the Lindsay Wildlife museum but see that was done below.
The drive in movie theater in Concord can also be a lot of fun.