Yucatan!!! Definitely!! I've enjoyed my brief time there, although in a car. Skip Tulum, or at least avoid the way overcrowded tourist area along the beach. Stay in town, if you decide to visit Tulum. I liked Valladolid very much. Skip Chichen Itza alltogether, and go to Ek Balam instead. I didn't make it to Merida, but heard is definitely worth a visit.