I grew up in Houston and still love it for a lot of reasons, but moved to San Antonio in '84 for a) easier lifestyle, b) reduced traffic craziness and c) MUCH better and more accessible bike roads. The Houston area's terrain is as you know, flat, and most roads in the nearby counties (forget Harris) are just paved over old rice field roads running at 90 degrees to each other. That adds up to boring, unless you like to run full throttle between intersections or too-sharp turns. However, here's one that I remember as being a lot of fun, and I hope it hasn't been ruined by the area's recent explosion of growth:
Guys I knew in early 80's who raced at College Station called this one "Racer Road." Take I-10 west. Just before Sealy, look for highway 1458 that runs north to Stephen F Austin Park. Landmark: there is a truck stop of some sort at that exit. After 2 or 3 miles of some residential stuff through the town of San Felipe, it turns into 10-15 miles of very nice sweepers before ending in the town of Pattison at highway 359. You can take 359 back down to I-10 for the return trip, or turn around and go back the opposite direction. Not a lot of miles, but doing it over and over again is a good workout and good for testing yourself and the bike.
Otherwise, I'd suggest you purchase a copy of "The Roads of Texas" and scout out the Sam Houston and Davy Crockett National Forests north of Houston, as well as areas around Nacogdoches and Lufkin. As in the Hill Country, however, watch out for the deer.
Good luck.