I agree with BillyandKris. Don't do Big Bend in the summer. But there's a special beauty in the desert. You can catch some of our photos on our
blog. We just came back from there, and the cactus were blooming like crazy which is beautiful. We did Davis Mts., and Big Bend and both were gorgeous. I'd get a reservation at the Rio Grand Village (FH or primitive) and stay there as you're close to the river and a birder's paradise. Then if you want to travel further east up the coast toward Aransas Pass, this is another fabulous birding area. The Hill country and Palo Duro are other great spots.
You might visit the
Texas State Park site for more info, and also
Top Ten Texas State Parks to get some more ideas. Buy a Texas State Park pass - it lasts a year and it will save you some $$$ along the way.