Schlitterbahn (German for "slippery road") is one of the premier waterparks in the country. They have a variety of chutes, slides, pools, water-coasters and other ways to have fun in the water. There is a five-story water fun house for the kids, a sports bar for the adults and a full service restaurant for the whole family. The park also has its own stretch of beach. Parking is free. Hours and dates vary so check the website (or call) before you visit. Admission is $56 for adults and $46 for kids 3 to 11 and seniors 55 and up.
Schliterbahn (German for slippery road) is a 'branch' of the water park by the same name in San Marcos, North of San Antonio. This version is not nearly as big, but it will do if you're into this sort of thing and want to slide down the tubes instead of swim in the Gulf.
There is also a restaurant serving seafood (The Shrimp Haus) which doesn't require the paying an admission fee.
It's a great water park to visit with your family, friends, etc. It can be for all ages. So many things to do there! Great water activities, great souveniers, great food.