One free day at Houston

Hey, I have one day to fill in my visit to Houston. I already have a day separated for the Space Center and Galveston.

So, the other day I think I can accommodate two of three places. The aquarium, zoo and museum of natural science.

What do you recommend?

Edit: Thank you everyone for your recommendations! I have bought the tickets for the museum and the zoo. And will avoid the aquarium like the plague.