Hi, my wife and I have been twice, our dive shop goes once every few months.
We went on a guided trip to Chetchen Itza (if you are into the Mayan Ruins). We took a shuttle boat to Playa Del Carmen (the main land/Yucatan peninsula) and met our bus there. I think there are some smaller ruins on Cozumel and horse back rides to them.
In Cozumel itself there is the typical shopping, eating, and nightlife (if you are into bars and clubs). There is a Dolphin swimming place which allows you to swim with Dolphins in their pool.
The shuttle boats are cheap and fast. You can catch tour buses in Playa del Carmen at or near the dock and travel to various destinations of interest. I think the last shuttle boat returned to Cozumel at 9 or 10pm.
Everything on the island "tourist end" is very walkable and easy to get to. The other end of the island has more beaches and college party houses.
I recommend doing some on line research for trips and compare prices. We bought our tickets ahead of time (I can’t remember who we went through). The Shuttle Boat tickets we bought at the window and got on the first thing going in the morning...be early it fills up quick.
Good luck and enjoy