Hi Spiro
We enjoyed cave Mammoth Cave but I don't think I would want to spend 2 nights there. I could be wrong, just my opinion, but I don't think there's much else to do. We enjoyed the Corvette Museum but have not been to anything else you mentioned. If you've never spent a day and night in Memphis, it is worthwhile to go to Elvis's home (Graceland) stop to see the famous ducks at the Peabody, and in the evening have BBQ at The Rendezvous http://www.hogsfly.com/ and walk Beale Street catching some jazz music and a few drinks. The only thing I'm familiar with in Nashville is the Grand Ole Opry.

We have been to Memphis several times and have eaten and seen everything you mention. Was considering Gus Famous and Deja Vu for a meal. We really like Blues City Cafe and the brunch at the peabody. My wife has eaten at Rendevouz and I haven't.

spirobulldog, not trying to talk you out of your agenda, but let me share something we loved in Kentucky, not far from Mammonth. We did some of the Bourbon Trail which was fun:
http://kybourbontrail.com/index.php/guide/
Here is my trip report:
http://www.fodors.com/community/united-states/bourbon-trail-and-shaker-village-in-kentucky.cfm
And we dearly LOVED Shaker Villiage:
http://www.shakervillageky.org/

I agree with cd regarding the Bourbon Trail and Shakertown. We visited Mammoth Cave in October and did the Grand Avenue tour(4 hrs) and really enjoyed it. Liked our room at the hotel within the park. It appeared to have been recently updated and was clean and comfortable. The restaurant at the hotel is very good so don't bother with trying to eat anywhere outside the park--fast food type places--the restaurant there is super reasonable too. The park is enormous and full of deer and there are some hiking trails if that is of interest to you. We stayed just one night there and then drove on to Bardstown and then to Harrodsburg--took day trips to several distilleries and Shakertown.

Shaker Village is great. Check out the brooms. Hand made there. I need to order a new one.

sorry, but distelleries would not interest us at all.

No need to be sorry spiro. I think you would really enjoy Shaker Village. We truly enjoyed staying at the Inn. Rooms are scattered thru-out the Village.

If you're stopping in/near Nashville anyway, it's worth a day (or more!) if you have it to spare. There's a lot more than the Opry - plantations/mansions, botanical gardens, an art museum, good shopping, eating, lots of historic spots and of course lots of music. Whether it's worth it to you just depends on what you're interested in and what sort of trip you want.

It's been years since we have been there but from reading the comments above not much has changed. The visitor center is right next to the lodge so you really don't have to drive anywhere while there. There really isn't too much else there. We did a cave tour during the day and then hiked around there in the late afternoon/evening. I probably would do another cave tour if you can -- not sure what is available. We really liked the lantern tour. In fact your wife, who I assume is the one who may not do the wild cave tour, could do a tour, like Frozen Niagra,during that time. I don't know of anything you would do that is exciting on the Green River. The hiking is not challenging at all, but there really isn't anything else to do there. It depends on the timing of the tours on how many nights you want to stay. We spent 3 nights because of that, but it was definitely a little long. If you know you want to do other tours, I would book them also. Not sure about that time of year, but we were surprized by the number of tours that were full -- schools and other groups frequent there.