The Texas Hill Country has a number of good rivers to fish; the Llano, Guadelupe and Perdenales are just a few. Search the internet for "fly fishing texas hill country", and for "Guadalupe River Chapter of Trout Unlimited". There are several websites dedicated to fly fishing in Texas that should prove helpful.
The Texas Parks and Wildlife website contains maps and other information about Texas waterways. You can also order your fishing license through their website if you need to.
Finally, I recommend Phil Shook's books "Fly Fishing the Texas Coast" and "Flyfisher's Guide to Texas".
Sounds like a great trip.
Tracy
Men and fish are alike. They both get into trouble when they open their mouths. ~Author Unknown