If you go to the ruins its an all day trip. We opted not to do them as we travel with our kids and I know they wouldn't have enjoyed it. Those who did go said that it was unbeleivable. From what I heard the bus ride is about 2 hours each way.
I think the shopping was better in Cozumel than in Costa Maya. At the port there is a shopping mall area in Costa Maya. In Mahahual there are vendors along the street selling trinkets. I found most of the stuff they were selling in Costa Maya was the same as in Cozumel, only it was cheaper in Cozumel. My daughter bought a bracelet in Costa Maya for $10. In Cozumel they were 3 for $10.
This year we plan to walk around in Cozumel, do some shopping and spend some time on the beach. In Costa Maya we plan to do the ship's tour to Uvero Beach.
Have a great time!