Ditto the recommendation for Canyon Lake, although I havenít been there this season to see the drought effects on water levels. Weíve stayed at Potterís Creek (U.S. Army COE) recreation area, which is very nice and has RV sites in the woods and on grassy peninsulas that extend out into the lake for awesome sunsets. Water/electric sites through reserveamerica.com website. If youíre a veteran, thereís also has Randolph Recreation Park at Canyon Lake or Fort Sam Houston Recreation Area (which have swim beaches and boat rentals). All of these campgrounds are along the lakeís north shore on FM 306 (I think itís IH-35 exit 192---head west).
There are campgrounds on IH-35 near New Braunfels, but I have no personal experience with them.
Consider Schlitterbahn Water Park in New Braunfels, repeatedly voted #1 water park in the U.S. New Braunfels Smoke House is a good restaurant for German food (they smoke their own sausage) in New Braunfels right at the IH-35. Consider a side trip to historic Gruene, TX----home of famous Gruene Hall with live music, shops, restaurants, antiques.
Oh yeah, and thereís a big Camping World near New Braunfels on IH-35 (more stuff!