One of the most popular water activities in Texas during summer time is tubing on the Comal River and Guadalupe River near New Braunfels. On the weekends, the Guadalupe turns into a great party river. It gets really crowded, but it is fun. While floating and relaxing on a tube you can enjoy a cold drink and have a nice conversation with people floating nearby.
The Guadalupe River has rapids, boulders, and chutes so floating is both fun and adventurous as well. Of course, everything depends on what part of the river you choose. Some of the best parts are the Horseshoe, the Chute, and around GrueneIf. Don't forget sunscreen, river shoes, a hat, and sunglasses for this trip. And be careful because there aren't any lifeguards.