Knott's Berry Farm. We went there a couple of weeks ago and loved it. It was a totally new experience with kids. I only went on one roller coaster ride the whole day and had a blast just hanging out at Snoopy Land and in the western-themed Ghost Town with the boys.
Wildlife Learning Center (in Sylmar) rescues wildlife and educates the public about the environment. It is like visiting a very small zoo but you get the chance to see and touch animals up-close. They do a "trainer talk" every hour on the hour and they let the kids touch the animal they bring out.