Austin/San Antonio things to do Options

[Mommy]

My husband's 14 year old brother is coming into town next week to spend 4th of July weekend with us. I need some suggestions of things to do.
Here is the list of stuff I am looking for:

• Affordable
• Fun for both him and us
• Not too hot
• Not a lot of standing
• Possibly less known so that the crowds won't be too terrible

Basically, we want to take him to the Schlitterbahn, but I don't know if I can stand being out in the heat that long. I won't be doing any of the rides of course, but it does seem like a huge waste for me to pay almost $40 to get in and not be able to do anything because I am about to pop. My husband is going to take him paint balling, but I want something we can all do together. I don't want to go any further than Austin or San Antonio (I live in San Marcos). Gas is too expensive. Any suggestions would be greatly appreciated, especially if they involve caves or spelunking. I think that would be neat.

[chook]

I used to work in Cascade Caverns as a tourguide, Natural Bridge is better.