Don did you get Holiday World ticket discounts? I have a bunch if you need some. I can mail them too you.
This is my neck of the woods too!!
http://www.bigsplashadventure.com/ There is an indoor water park in French Lick that has some deal with Holiday World to stay there. There is also a train ride in
http://www.indianarailwaymuseum.org/
Squire Boone Caverns
http://www.squireboonecaverns.com/ is fun, that would be near Corydon IN. on your way to Mammoth might be worth checking out. Not an all day thing at all. . There is a fun restaurant up there that you can look out on the river in Corydon.
http://www.theoverlook.com/
I didn't hear this thrown in or how long you will be there but the Louisville Science Center and zoo is fun. The zoo has a Dinosaur exhibit there now, if you have any dino fans.
Also near Mammoth Cave is Dinosaur World
http://www.dinoworld.net/ and Kentucky Down Under
http://www.kdu.com/
Let me know if you need anything while you are down here. Or any question!!