Another vote for the Yucatan. No need to limit it to that though, you can keep heading south into Belize, Guatemala, Honduras, El Salvador, Nicaragua, Costa Rica, and Panama if you want. I did a 9 week trip there last year and used the tent exactly one night. Cheap rooms the rest of the time, including hostels, hotels, and a couple of nights couchsurfing. I was there Oct-Dec and the only place it got a little cool was crossing the mountains in Panama. Other than that it was either warm or hot, and will probably be that way in Jan-Feb as well. Great people everywhere, lots of great sights, inexpensive food and lodging.
CGOAB link in my signature.