I would pass on Chichen Itza (Chicken Pizza) and go to Coba. You can climb to the top, it's closer and the view over the forest canopy is great. Rent a car and leave early to beat the hordes arriving by bus.
Then definitely go snorkeling in the Cenotes. Amazing. I'm not a tour guy, but I highly, highly recommend
Altournative Adventures. You get to ride in a Unimog - totally cool - and they support local sustainable projects instead of the big commercial crud. Our trip had kayaking, snorkeling in a bay and in two cenotes, the jungle crossing in the Unimog, a complete blast.
