I haven't been there in years, but the Tennessee Valley RR Museum is a great place. Short ride, turntable action, steam. Then there's the Chattanooga Choo Choo--hotel converted from the RR station.
There is an enormous HO layout on the second floor of the station, http://www.choochoo.com/modelrailroad.php. It was fantastic when I saw it in '87, and I'm sure it's bigger-n-better today. Hope you have a fun time whatever and wherever.