Here's what we did on our May 10th Cruise:
Key West - bought tickets for the trolley online and saved $4 per person over the
DCL price. It is very easy to find the trolley booth downtown (it is right next to the Aquarium) and that took us all around the island. You can get on and off as you please. We got out at the Southernmost point stop to see that and the Butterfly Conservatory, then walked up to the Hemmingway House, Up Duval street to shop, and later hit the Aquarium and shops near the ship. Very easy to do on your own.
Grand Cayman - We did the DCL tour which was Snorkeling with Stingrays and Discover Cayman. First stop Hell, then the Turtle Farm and then finally the Stingrays. We did the early excursion which met at 7:50 and got back to the pier around noon. Plenty of time to do some shopping and get back on board before everyone else.
Cozumel - we did the DCL tour called Cozumel Ruins and Beach Tour. These ruins are right on the Island and the whole excursion took about 4.5 hours. The other ruins tours go to the mainland and take 7.5 hours - a little too long for our taste. The tour we took was great with a short ride to the ruins, a walk through with a guide, time to shop, then a beautiful ride along the coast. Last stop is the Playa Sol beach where we had an hour to shop, play, go in the water, whatever. Then back to the ship. This tour left at noon so we had time to shop in the morning, grabbed a quick bite to eat on board, and got back on the ship in plenty of time to get ready for early seating dinner.