Taco Cabana is the texas version of Taco Bell, and its AWESOME. I also like Whataburger in the fast food category.
Give Chuys a try. There are a ton of places on the riverwalk that are really good and fun. Taco Taco supposedly has the best taco in the US.
Walk into any higher end hotel and ask them what the best tex mex place is . . and they will give you a bunch of recommendations.