I came across this cool promotion recently. In partnership with Time Warner Cable, many local museums will let kids 12 and under in for free with a paying adult. To find out more about this and to download the coupon that gets you in click here! If you're over 12, it means that if you pay to get in you can bring a younger sibling or perhaps someone you're babysitting in for free.
The San Diego Museum Council's website has even more information about the program and other museum related activities happening in San Diego.