Galveston is fairly decent for cruising up and down the seawall. If you're a serious roadie, you may run out of island before you run out of energy.

THere's a book at local bookstores about the "Best Bicycle Rides in Texas"- you might peruse through it. Your library may have it as well.

Of course, you can do all kinds of riding here in the Dallas area or the Houston area, but I'm not aware that any really has any advantage over San Antonio, either.