Pretty much eat, drink, and watch live music. That's Nashville.
If you are a country music fan I think a tour of the Ryman Auditorium is worth it. Only takes a couple of hours max and it's right there on the main strip.
Just go up and down Broadway and pop into anything that peaks your interest. Most of the live music is incredible… one of the better shows I've seen live was at a small dive bar on Broadway at 12 pm on a Tuesday. It's hard to not have a good time out there… just be ready for a lot of people.