juju, in order to determine the best excursions for you, it would be helpful if we knew what type(s) of excursions you were seeking. Are you interested in beaches, snorkeling, shopping, Mayan culture, general sightseeing, etc.?
Island Marketing is who handled Nachi Cocum in Cozumel and I have read (I have never used them)many favorable reviews about that beach for those adults who want a more peaceful atmosphere.
If you will also tell us which ports, perhaps we can be more specific.
Recommend you look at CCL's website for tours. I would advise against taking an independant tour. If you book with the ship and the tour is running late they will wait for you. If you book independantly and you're running late, you'll miss the ship.