When it's crackling hot outside, the Downtown Houston Aquarium is a nice and cool spot to hang out. For about 10-bucks for adults and 7 for kids, they have different "adventures", including this beautiful "Sunken Temple" exhibit with my faves, the Pufferfish. Be sure to check out the Aquarium's resident octopus, who is a pretty clever fellow... and their awesome electric eels.
Cavern Tours Georgetown, TX Central Texas has cave after cave, the area is known as the Hill Country and you can caverans and caves all over the area centering around Austin. I have been to several and enjoyed them.