We really enjoy the drive-in theatre in Franklin, KY, depending on where you are in Nashville, it may be very close. I don't think it took us more than 25min or so to get there, and it is $9 a carload and on the weekends they have a double feature. Plus a kickin' (reasonably priced) concession. We went there for the first time a few weeks ago and are looking to go again real soon (like this weekend). Also it seems to be a tad bit cooler there than nashville so it was a very pleasant evening.
Index
Also, during the summer at the area regal theaters on Tuesday and Wed, there are free family movies at 10am. There are 2 choices each week.
Enjoy!