I happen to know a lawyer and a stalker who know about both of those cities!!!!
The Frist Center for the visual arts.
Outstanding kids section! Kids are free too. I'm not sure what exhibit is on now or later for this summer, but I'll look it up.
do NOT miss the kid section. it's called Artquest and is on the second floor. there are always several exhibits that relate to the exhibit downstairs -
like when African cloth art was on display, the kids got to make their own print on a piece of cloth. that's probably still there.
when it was stained glass, there was that cool transparent paper that you can color with markers and it looks like stained glass.
they have figure drawing, a big piece of plexiglass so two people can draw with dry erase markers and make a collaborative art work.
and the kids get to keep an online portfoilo (probably not too beneficial for a one time guest) of all their work.
if they're into science (and being your kids, how could they not be?!?!?!) the adventure science center is TONS of fun!
I'm guessing you'll be driving through the knoxville area too - wonderworks in pigeon forge is both science AND art kid friendly exhibits - and it's not too far off the interstate.
if you want to go to the aquarium in gatlinburg, that'd be cool, but it's a good ways off the interstate.
I'd forgo the country music hall of fame in Nashville unless you're just REALLY desperate. it's way too expensive. I've never been cuz I'm a cheapskate.
not sure which guitar thing your dad wants to see. there's some big one downtown and then the gibson showcase at opry mills.
at any rate, a certain stalker works in downtown nashville and wouldn't mind to have lunch with the twinkies.
or you can be too cool like peej.
