I've only had a jeep in Houston for about a month so haven't had time to check any of these out...yet. That said, these three seem to be w/in easy driving distance from the metro Houston area:
Down South Offroad (posted by deathphoenix)
Creekside Edge Off Road Park Splendora, TX
(which looks close by in Splendora, TX) and
General Sam's Offroad Park - Huntsville Texas - ATV Riding Trails & Camping
which is a little farther north in Huntsville but also looks great and they have trail maps posted on their site.
I plan on getting out to check them all out while the weather is getting close to perfect. I've probably done the same as everyone else and checked out the webpages and you tube but any live-experience feedback and recommendations are greatly appreciated.
Once I get those under my belt with minor damage I can start working towards the hill country parks