Are you planning on tubing down the river? We figured out the least expensive way to do it. We bought our own tubes, and paid 7 dollars to park in the big park right next to Schlitterbahn. You get in the water there and float for around 2- 2 1/2 hours, get out at the last exit, and its about a 2 block walk back to the entrance point. We camped on the Guadalupe side, which was nice because no one was on it so it was really nice!

Edit: If you EVER get the chance go to Big Bend National Park. It's a long drive, but man, it is awe-inspiring!