Off the top of my head, a few ideas...

I think Amtrak (the train) is preferable to the bus system.

I went to Europe when I was 17 and came to the precocious conclusion that (for me) Paris was "nothing" if I didn't have someone to share it with. (My best friend was with me.) So, I would strongly urge that you meet up with friends/mudcatters along the way.

If you're interested in geography, think about mountains and rivers. You can see the Mighty Mississippi, etc. Once again, when you narrow it down, a local Mudcatter can give some tips. I was always interested in the history of places I visited. The web can make your research a lot of fun.

I could get you from Nashville to Louisville (about 4 hours), but this is in the central part of the country, quite beautiful, but not hardly on the way to anywhere else. My guest room is hot in the summer, but available.

You can't see it all, but the planning sure is exciting. Let us hear more as time goes on.