I just returned yesterday from my first cruise ever. It was one of my best vacations. I had such a great time that I didn't really want to get off the Majesty when it was time to disembark.
Anyway, in Nassau, we didn't sign up for excursions. We just took a cab to the Atlantis, bought a tour of the Dig and the resort itself, went back to the interesting spots, and then had lunch at one of the restaurants. All this took 3 hours. We then took a cab back to main shopping street, which is just a few blocks away from the wharf where the Majesty was docked. So it depends on what interests you. If you want to see more of Nassau, it might be good to join one of the city tours. If you want water activities, join one of the organized tours or just head to Cable Beach. I think Coco Cay is the place to be in the water and just lay around on the beach. My teenage niece and her friend went on the Waverunner and enjoyed the experience tremendously.
As for Key West, getting on the Conch Train or Trolley tours gives you a good overview of the city, if you've never been there before. One of our new-found friends did the snorkeling excursion and came back with very positive reviews.
I traveled with my sister, her daughter, and a friend. I'm in Arlington so if you have any other questions while the trip is very fresh in my mind, you may e-mail me at firstname.lastname@example.org