We did the Tulum ruins in Cozumel and the stingray city tour at Grand Cayman. They both were well worth the time and $$. We did the stingray tour with Captain Marvin's and can highly recommend them. The Tulum tour we did with Disney and would do that again--there is a long bus ride involved. The only thing I wish we had was more time to shop!
I second the excursion with Captain Marvin. It was MUCH less expensive than doing it through the cruiseline and you got a much better tour because of less people. We got to hold, feed and wade around with the stingrays for almost an hour. They also brought us to the Caymen Rum Factory and the Turtle farm.
I'm not sure what else there is to do on the island but I'd go with Captain Marvins.
We rented a car for the day which was an adventure as they drive on the left side of the road and the first thing we came to was a giant roundabout which made for an exciting introduction to driving on the wrong side of the road.
We drove out to Hell, went to the turtle farm (my wife was very afraid of the turtles but she did at least let me take her picture touching one), hung out on the beach, and then drove out to a very nice lighthouse themed restaurant right off the beach for an amazing lunch.
This was our favorite port of call on this cruise (Holland America Zuiderdam)
Maybe you folks can tell me though. Does gettin' two womenfolk mean you're the winner or the loser? ~Mine Train Through Nature's Wonderland