In your position, then, I would do St. Augustine, Savannah, Charleston, and Philadephia. I like walking around cities as well, and I really enjoyed Philadelphia for that reason. I'd try to fit in New Orleans if it doesn't mean too much extra travel time but I suspect it would. I don't know about St. Augustine but the others all have hostels.

It's a valid point that you can have bad seat mates on planes and trains as well. But on trains you can usually move away from the person, and on planes, that's three hours instead of 10+ hours next to a drunk. Trains are more expensive but generally that means you'll be riding with other tourists or business folks. (admittedly, I'm a little biased- buses make me motion sick while trains and planes rarely do).

One solution to avoiding a possible problem seat mate on a bus is to not sit next to a window and take your chances on who ends up next to you -- be proactive and sit next to someone who looks like a good option. And sitting near the front should help as well. But if my experience is any indication, having a bad seat mate on a bus is unusual.

I've been traveling on Amtrak for 15 years now. On my first Amtrak/ViaRail trip October 2001, I had a seatmate that I count as one of my best Facebook friends.
On the train if you do have a bad seatmate, you do have the option of asking the car attendant or conductor for a new seat. You also have the option to go to the lounge car for a couple hours. The snack bar is downstairs in the lounge car.
The dining car is usually the next car forward from the lounge car.
I haven't been back to New Orleans since Katrina so I don't know about hostels there anymore.
The biggest HI hostel in the US is in Chicago. I do like HI hostels more than any of the others.
Tomorrow night my wife and I get on the Coast Starlight to Sacramento where we get on a bus to Stockton. In Stockton we get on the San Joaquin to Bakersfield and another bus to where DS, DIL and 3 Grandkids live.

You can get off the train in Jax and take a bus to St Augustine. Very pretty with all the lights at Christmas. The bus takes 1-95, amtrak runs along it but a bit more scenic. I know it stops in Savannah and stopping in D.C. would be great.