A few general comments on what you asked:
1. I really enjoyed Thailand and found it pretty easy to tour and not too expensive assuming the air miles got you there first [knowing nothing else that what you posted, this would be my preference].
2. I've also toured in Cambodia and Vietnam. In comparison, I found Vietnam a bit more hurried and chaotic and Cambodia less developed, though I'm guessing this also depends where you ride.
3. As far as touring inside the US goes, Amtrak from CHI to NOLA is pretty simple. However, at least the roads I picked were a bit narrow and busy. There might be better choices. Some of the roads I picked in Florida were also busy, though I found at least the interior (away from Orlando) to have some quieter choices. The nights in the Southeast will be warmer than going further Southwest where the lower humidity also results in more of a day/night swing.
4. Personally I also enjoyed cycling in Baja Mexico. It took me longer than the two weeks you have allowed, but riding down the Baja Peninsula was a trip I enjoyed and there were plenty of tour reports I found of others doing a similar trip.
5. My own Christmas trip this year is likely to be starting in Brownsville, TX and cycling north up to Abilene. South TX is ok over that time, though slight possibilities of freezing/icing roads to watch out.
But even with all that if you can get there with air miles, I still like Thailand .