Outpost Armory off of I-24, exit 89 (Buchanan Road) is a great one. It's owned by Barrett of the famed 50 BMG. Lots of nice 1911s if that's your fancy. Not much used though. I stop by every time I head to Nashville from ATL which is about twice a month.
Franklin Gun store off of I-65 has a HUGE selection but prices are high, however, if you're just there to look, it's good.
Nashville Armory just opened up and it's a nice range if you plan on doing any shooting. Probably the most state of the art range I've been to. Their gun selection is only ok but they are brand new and building stock.
Guns and Leather is nice also. It's a little out of my way so I haven't been in quite some time.
Enjoy your stay in my favorite city (my hometown too)!