This place is a little hard to get to - because it is on mainland Mexico accross from Cozumel. But if you ever see a tour to Ixcaret offered on your cruise, I recommend going there. It is a great natural park where you can swim, see wildlife, swim with dolphins (extra cost), tube through caves, etc.
Also - if you can ever get to the Mayan Ruins Chichen Itza (near to Cancun) I recommend going there. In the old days you could get a plane there from Cozumel, but they dont offer that anymore. Too bad, it was a great tour offering.