My first recommendation would have been to take another ship (MV Grand Princess has some annoying layout flaws) -- but it's too late to cancel and rebook, so make the best of the situation and enjoy the cruise anyway.
As far as your ports of call is concerned, Grand Cayman is a personal favorite. You probably will feel right at home there since you speak the language, etc., and there's no problem going ashore on your own. Seven Mile Beach (which really is only 5 1/2 miles long, in spite of its name) is one of the best in the world, readily accessible by taxi, and has stations that offer locker, chair, and umbrella rentals and comfort facilities in addition to the beachfront resort hotels. George Town also offers some of the best shopping in the Caribbean, with a lot of quality merchandise, in a duty-free area right across the street from the tender landing -- but mentaly add 25% to quoted prices before agreeing to any transaction since the Cayman Island dollar is pegged at $1.25, making prices sound deceptively low. While shopping, be sure to stop at the <a href=http://www.rumcales.com> rumcake stand </a> for some tasty souvenirs, too -- the coconut rum cake is a personal favorite, and the prices there are much lower than on line!
Grand Cayman also has several unique attractions. The excursion to Stingray City is very popular, especially among the younger crowd, but the rock formation at Hell and the world's only sea turtle farm (yes, they really do raise the creatures for food!) are equally unique. I'm especially fond of the "Sea and See" tour, which combines a tour of the island's major attractions (Hell, the turtle farm, and a photo stop at Seven Mile Beach) with a harbor cruise in a "semisubmersible craft" (which really is a boat -- it does not submerge at all -- in which you sit in a special compartment below the waterline and look out underwater through windows in the hull) to visit the wrecks of the SS Callie and the SS Balboa on coral reefs nearby. You also will see plenty of tropical marine life since the waters off George Town are teaming with it!
The island of Cozumel offers a difficult decision -- whether to stay on the island or to take a more ambitious excursion to the mainland. The excursion to the Mayan ruins at Tulum is fascinating, but it is a long day. This excursion takes the ferry from San Miguel de Cozumel to Playa del Carmen, then boards buses for the ride to Tulum (which is pretty long in spite of the new limited access divided highway that runs along the coast), then reverse the trek to return to the ship in the afternoon. On the other hand, the ruins at Tulum are quite extensive and the guides -- who are Mayan -- are very knowledgeable, so the excursion definitely is worthwhile. The excursion to the archeo-ecological park at Ex Caret (pronounced "etch carray") -- which apparently combines Mayan ruins and watersports with an ecological perserve -- has similar transportation arrangements, but it also draws very favorable reviews from its participatnts. If you opt for either of these excursions, though, I recommend bringing a liter bottle of water from the ship and drinking from it frequently to prevent dehydration. Typical temperatures in Tulum are in the upper 90's even in the winter and there is little shade in the archeological site.
If you want a less ambitious tour, there are plenty of options right on the island of Cozumel. The tour that combines the Mexican cultural show in San Miguel de Cozumel with a visit to the Mayan ruins at San Gervasio is an excellent choice. The cultural show is one of the best that I have seen. The ruins at San Gervasio are not as extensive as the ruins at Tulum, but they are much more significant because the island of Cozumel is a holy place in the Mayan religion.
Unfortunately, I can't offer recommendations as to what to do at Costa Maya, which is a recent addition to the Western Caribbean itienrary.