we've been to cozumel via cruise ship 3 times (the most recent in december). It is a very "user friendly" island. Our favorite thing to do is to rent a car for the day (about $60 if you rent ahead of time at budget - the office is right at the pier), and explore the island. That way you can bounce from beach to beach, see the rougher atlantic ocean side, etc... It was a really good time
Whatever you do have a wonderful trip!