Cabrillo National Monument, San Diego
Address Cabrillo National Monument, Cabrillo Memorial Dr, San Diego, CA 92106 Map, Tel (619) 557-5450 Call, Website
You can drive up to the visitor center and learn about Juan Rodriguez Cabrillo and watch the boats in the harbor, which can be everything from sailboats to Navy cruisers.
Then you can tour the Old Point Loma Lighthouse and go through the exhibits showing how lighthouse keepers used to live. These exhibits are not extensive and so most kids can wander through quickly.
But the best part is driving down to the tidepools. Make sure you go in low tide--easier in the winter than summer--and the little ones can spend hours looking at the tidepools. There are all sorts of boats out at sea, pelicans and other birds flying by, all sorts of things in the sea.