My listing of things to do topside in Cozumel (in no particular order):
1) Do a jeep day around the island- My favorites are Dzul-Ha (now Money Bar), Chen Rio, Coconuts, Mezcalitos, and Palancar Beach Club. Everyone has their own opinion on jeep day favorites, though.
2) Get a massage from Sally Hurwitch.
3) Go the San Gervasio ruins. They aren't Chichen Itza, but it is not a half day drive, either. It is hot there, but I was pleasantly surprised by how cool they were.
4) Go to El Marcado. It is a cool way to kill an afternoon if you are a foodie.
5) If you are really a foodie, do the cooking class with Concina Con Alma. You can find the link on Cozumelmycozumel.com. Josefina is a super host and does an excellent job. The food is first rate. We have used her tamale recipe many times.
6) Go snorkeling. I know it sounds crazy with the diving we all do, but seeing the reef from the top side looking down (such as at Columbia Shallows) is a pretty cool way to spend an afternoon. It is amazing how different the reef looks from above.
7) Read a book. Take a nap. Drink in the afternoon. All excellent activities, IMO.
That should be a good start.