Also the Dallas Zoo is nice (although if you have your LTC you can't carry there or the Perot museum).
The Dallas aquarium was not worth the money. It's more of an aviary than an aquarium. The aquarium in the Grapevine mall was honestly better. And if you go to the aquarium there you may as well sto pat Lego land.
My wife and I took our three year old to all three of those places over last spring break and he loved it!