The first time my wife and I were in Cozumel we booked the Passion Island excursion (Isle de Passion) and it was great. First you take a bus..oops...luxury motor coach
, then a boat ride to the island where you are served a huge lunch wich was very good, then hang out on the beach and relax or play volleyball.....the beach is beautiful and the have a good portion of it has palms and plants and what not. Very nice.
The second time our two sons (16 & 20) went with us so we booked the ATV tour wich was great fun. We went to a beach and a lighthouse and the guides were great.
We didn't do the Mayan ruins but we did see them in Costa Maya and they always prove to be fascinating.
Also we almost always book through the ship. It costs a little more but it's well organized and the ship can't leave if your tour is late!
You'll have a ton of fun whatever you do if you have a positive attitude.