Vacation Advice for Playa del Carmen and Cozumel?

My wife and I will be heading to Playa del Carmen and Cozumel for about two weeks at the end of February.

Our plan is to spend the first half of the time in Playa, and to go from there to see the major archeological sites: Chichen Itza, Tulum, maybe others. This would be the "adventure" part of the vacation. We'll probably want to go Scuba diving in the Cenotes when we're there. We're definitely looking for other things to do during this half of the vacation.

The second half, we'll spend in Cozumel. I'll be diving a lot. Laura may dive, or may spend lots of time sitting around on the beach with a cocktail in her hand. We're considering an all-inclusive, in part to just relax more, and to have a "vacation from our vacation" while still on vacation.

We're looking for things to do, places to see, and meals to eat for the first half of the vacation; and we're looking for recommendations on where to stay for the second half.