We have done both
three westerns one DCL, one RCCL and One DCl w/ Costa Maya.
We have done the jeep ride in COzmel- DH & DS # 2 loved, DS # 1 and I went to Mr Santos- juat a beach, on RCCL we went and did a snuba dive, it was fabulous, every one from 9-60 could so it with no dive experience, the last DCL we did the Fury catamaran- lost of fun- no kids, snorkling not that great, but the boat ride and the partying were!!!! I am glad my kids were not here because we drank too much, and I don't want my kids to ever see me being that silly....
Grand Caymen- both times did Stingray city- fun with the kids, don't think I would go with out, once you have done it. Took a taxi to Hell- that was interesting.
Key west- walked around and went snorkling.
Costa Maya is beautiful, not as commercial reminded me of Guam.
We did the stingrays the first trip to Grand Cayman (Jan '07). We did Seven Mile Beach after that (right at the gray hotel - I think it was the Hyatt or something).
When we went back in Oct '07, we just wanted the beach because it was just beautiful. I looked online and everyone was saying to just take a cab to Public Beach which is on Seven Mile. Karen, Neil and Ellis came with. There were facilities and restaurants right there.
As for Cozumel, wed did Mr. Sancho's (or the one on the other side of the point there) in Jan '07. When we got there in Oct '07, it was really windy and the waves were crashing up on the walk way in town.
Here is a picture Andy captured for me that day:

You can see how grey it looked. You can't see the waves, but they were bad. I think almost every water excursion got cancelled that day. We walked around town a bit, stop at Pancho's Backyard (always stop there when we go), and then back to the ship.
When we went to Playa in '04 for Suz' 40th, we went over to Chakanaab (sp?). We really like it there. We saw the snuba people right under me while I was snorkeling. The guide probably pointed out the whale swimming overhead.
