DFW if you're really into texan culture aka country/wild west. Ft Worth has cowboy landmarks and its a train ride away from downtown dallas. you could rent a car in Ft Worth. Both cities have nightlife so time your visit with a good concert. i'm going to see steely dan and doobie brothers this weekend. there are a few decent museums between the cities.
Austin,TX lots of nightlife and underage (18) poon. maybe historical significance. you can go have BBQ in lockhart, TX an hour away where the famous smokehouses are. they only serve on certain days and they run out of BBQ at the end of the stretch, so go earlier during open days rather than later.
San Antonio - lots of nearby history (alamo and san jacinto?) - riverwalk, lots of mexicans. oh wait that's the whole state
