E4NASH ... no one answered you about Tulum so I will. It is very much worth going to see, it's impressive, I loved it and have always wanted to go back.
The down side is that you can not get to it from "Cozumel" because it is on the mainland. You must take the ferry to the mainland, and then about a 1 1/2 hour bus ride to Tulum. If you are not on "Carnival's" tour and you are late getting back they will not hold the ship for you. The tour to Tulum is a long tour, I don't think Carnival offers it any more because of this. If you use a private tour operator, they may promise to get you back on time, but if something goes wrong ... your are stuck
There are plenty of things to do in Cozumel, the dolphin swim is really a great thing to do, check out Chankanaub (sp??) park there are so many things ot do there alone, snorkle, eat, drink, dolphin swim and lots more.
Have a great cruise!