My very favorite place to bring my girls is Pretend City in Irvine. It's a place where they are free to roam and explore freely, to use their imagination and learn new things by actually doing them. They can be waitresses in the restaurant, police officers at the police station, a doctor or nurse in the clinic, farmers plowing in the field or a construction worker building a kid size "house". They have watched chicks hatch from their shell, made pinwheels that really work, listened to many an acted out story by the staff and put on their own theatrical shows on a real stage. I love the chance to spend this time with them because we can just have fun. There is not the usual pressure for me to come up with ways to entertain them or worry about cleaning up the my house after the fun is over.
It's a real bargain if you buy a family pass like we did and go frequently. Pretend City is great for kids 2-6ish but if you have sibs that are older or younger, there are activities for them there as well.