I think you will get a lot of people responding with Disney World, which I agree is an awesome place. We started taking my oldest when she was 1.5 years old and she got older, the other kids followed suit. Now they are 6,4,2, and 1 and they love it!
But, for a different answer, I would suggest San Diego. The zoo, wild animal park, and beach are all right there. The zoo and animal park are huge and the kids have so much fun seeing all the animals and strolling around. The beach is one massive sandbox that will entertain kids (and dads while mom is catching a break!) for hours. About 45 minutes away is Lego Land, a theme park perfect for kids 8 and under with rides and lego building stations. If you really want a Disney kick, you can always drive north to LA a few hours and go to Disney Land and catch all the characters and rides there. It is smaller than Disney World, but in my personal opinion is much more laid back and relaxed.