Landa Park in New Braunfels, Tx. We highly recommend this park for families, especially the Aquatics Complex (there is a small admission fee). We had gone down to New Braunfels, a town south of Austin, for their annual Train Show. The boys really enjoyed the elaborate layouts and because it was a small show, it didn't overtax them. The admission to the show included a ride on the mini-train in Landa Park, which the boys loved. Then we spent the rest of our time (until naps were absolutely required) playing at the Aquatics Complex.
There are two things I really like about the AC. First, it has a big, spring fed pool (like Barton Springs except 72 degrees instead of 68!) with diving platforms, a giant slide, a zip line and a rope swing. I like that they haven't taken all the fun out of this area by disallowing diving, etc. I can think back to the thrill of diving into a pool and it's that pure joy feeling all over again - when was the last time you were at a pool that allowed diving? I miss it.
The second think I like about the AC is the big areas with 6 inch - 1 foot water depth. The youngest toddlers are able to splash around, not automatically in over their heads. The slides in these areas are great for the toddlers and still fun for the preschoolers. There are big mushroom waterfalls that they can run in and out of. Our kids played for the longest they've ever played at the pool and were so happy to be self-sufficient in the water.
This is the second year we've done this combination of activities and both times it has been great family times. I'd say it's totally worth the mini-road trip!