Kid City @ Reuben Science Center
I just became a member of the Reuben Science Center, located at Balboa Park, in front of the big fountain.
It is a fantastic place and the membership is so well-priced! I actually got the family membership without the IMAX tickets which was about $20 cheaper.
Aside from the floor gallery of wonderful science displays for adults and kids alike, they have a FANTASTIC play, explore area upstairs called KID city. It's an area for children 5 and under and there is so many nifty things for the young ones to do inside....even the parents enjoy participating in the activities.
There's a computer station with lots of fun, learning activities, a play house, supermarket, blocks, play food, and balls everywhere to feed into these air tubes.
The kids LOVE it. It is such a nice place to take the kids especially on a cold or hot day. But be warned that this place can get very busy on Free Tuesdays as well as during the summer with all the kids out of school.