As far as the 8-hour layover, you could go visit the AA C.R. Smith Aviation Museum.
http://www.crsmithmuseum.org/home.htm. You should definitely check out Terminal D, the new one, which has some good dining choices.
In terms of leaving the airport, you would need to take a cab to do much of anything or else rent a car for a short time. There is a lot of shopping north of the airport at a place called Grapevine Mills (google it for more info). You could also drive to Ft. Worth and take in a couple of musuems there, including the Kimball or Modern or Amon Carter (
check here).
HTH. Feel free to respond with any questions.