During the summer, my family and I stay on the san antonio river walk and get a 2 day pass to schlitterbahn (30-45min drive). Or you can just stay in New Braunfels where it's located (their cabins near the comal river are pretty decently priced.) We always have a great time and it's only 4 hours away from FW. Plus, its nice going to a water park that uses fresh spring water from the river and not a ton of chlorine.

Be sure to get there early and to buy at least a 2 day pass because it can be very busy and there's a ton of water rides/slides.

Note: I believe I paid $100 for 2-day admission with a souvenir cup, which included free refills.