R.R.
I have a friend with 9 grandchildren in Nashville (number 10 due any day, all from the same parents) who visits them a few times a year, and they LOVE the zoo and aquarium. She gave me these sites to share as well:
http://nashville.about.com/cs/familyactivities/a/summerfu...
http://attractions.uptake.com/tennessee/nashville/cheap_v...
http://www.grandparents.com/gp/events/nashville.html (Not just for grandparents)
http://www.visitmusiccity.com/visitors/funandfree