We travelled to Mexico for the first time this year, from Dec 17 to Dec 24. I'd have to say that the most amazing thing we did was dive in the cenotes. This is a network of underground rivers and caves running throughout the Yucatan penninsula. It was amazing. We dove in areas where there was NO natural light, the light in your hand was the only light you had. We popped up in an air pocket beneath some roots from the trees growing in the rocks over our heads. While we were in there, we heard the rumble of the other tour vans leaving the area, RIGHT OVER OUR HEADS! Carlos, our guide told us that the rock above us was probably about 5 feet thick. Amazing.
He knows so much about the area and the history - both geologic and cultural. I've got his contact info if anyone wants to look him up.