T.E.
The caves near San Antonio are lots of fun, plus they have an area where you can rinse bags of dirt with arrow heads and special stones in them. You purchase the bags there and they are all guaranteed to have certain objects mixed in. I believe the term is sleucing sp?(you put the dirt in a screened box and water runs through it to wash the dirt away.) Everything you need to do it is right there. My kids loved doing this for years and had more fun with it than the caves themselves. This is also near a drive through animal park -probably too hot for that to be enjoyable this summer.
Another fun activity is Aquarena Springs in San Marcos. It's right off the interstate and has glass bottom boat rides. They run fairly often, aren't too expensive and are a fun/different thing to do. We have taken our kids and niece and nephew there.
San Antonio also has an awesome children's museum. Have fun!
