San Antonio is hot enough. When you get out to the west, you get hotter if anything, and that would be something to consider if heading in that direction. That's one reason I thought of the coast first.

Beavers Bend is nice. I don't know about the riding. You've got maybe a couple of miles or more of riding in the park. Once you get out of the park, it's pretty much straight asphalt roads as I recall. There are lots of rental cabins, check into them.

That "best cycle routes" book has one route listed on Padre Island and one in Corpus Christi, as I recall- neither is terribly long, but a good morning outing.

I was down in Falfurrias a while back, and that looked like some great riding to me. Not sure what there is to do in the area (check in Kingsville, they had a big visitor center there), but the roads in that area tended to have very little traffic, and different scenery from Dallas- basically flat. I drove the loop around Benavides, Realitos, and Concepcion- just very little out there (meaning, take water).

I've been in Balmorhea twice, I think. One time was right after a tornado had gone through Saragosa. There were irrigated cantaloupe fields in the area, and off accross the fields, you could see several dust devils whirling away. I kept wondering if one of them would make the next tornado. But it's hot out there, too, and shade trees are few and far between. It's pretty in a desert sort-of-way. I wouldn't mind riding out there myself, but would try to pick cooler weather in fall or spring.