The zoo. We buy year passes every year. It gets us in the Audubon Zoo, Aquarium and Incettorium for a year.
The zoo is my favorite. The Aquarium of America's is nice, needs to be updated. However we love the new bird room. It is like a sanctuary, the can move about us freely and we can feed them. The kids love it. At the Incettorium they have all kinds of different rooms, but I think the butterfly room is awesome! Plus there is the added bonus of the cafeteria, they serve different types of bugs on the menu!
When we do the Aqurium, we make a day of it and go hang out in the French Quarter as well.