Full-scale replica of the Parthenon and statue of Athena Must be seen to be believed. Built for Tennessee's 1897 Centennial Exposition. More info: Wikipedia article, Google map
Atrium gardens in the Gaylord Opryland Convention Center Don't be put off if country music isn't your thing (it isn't mine either). The gardens in the four atria are what I'm recommending. The convention center is big so don't be shy about asking the front desk for directions to the atria. More info: Google map
You definitely need to do an evening at Roberts ! Maybe even buy some boots off the wall!
For the Carnegie hall of bluegrass, you've got to check out The Station Inn. Bluegrass is the techno of country and regardless how much you like or dislike the genre, you'll be amazed at the talent of the musicians.