The resort included two free tours in our package and, of course, we had to go see Chichen Itza.
It's a long drive there and back, so you'll have to pretty much write off the whole day, but boy, is it worth it! I mean, okay, there's about a dozen or more tour groups schlepping around too, and it's HOT, but if you can put all that aside, it's a must-do.
The pyramid is awesome and awe-inspiring. If you can stomach heights, climb up to the top -- the view from there is amazing. Climbing back down is the scary bit, but everyone does it on their behinds, so I didn't feel too bad (see photo).
We were particularly thrilled with the snake associations (my fiancé is MAD about snakes) and we have tons of pictures with him at each snake head, carving, etc. I am also ALWAYS amazed at how advanced all these "primitive" civilisations were . . . they had an observatory all those thousands of years ago and knew exactly when the solstices and equinoxes were. Too cool!
We also had a lunch included with the tour, but it was about as uninspired as you can imagine at a place where hundreds of tour buses stop every week! Oh, also, if you can, arrange some sort of breakfast pack from the hotel or just buy some stuff to eat, as you'll leave early and may miss breakfast (not a good idea when the only stop is a tourist "trap" and they charge a small fortune for a bag of Doritos).