I'm the family cruise editor here. We went on the Monarch of the Seas many years ago with my daughter and she loved it. We just went on Royal Caribbean's Adventure of the Seas which my daughter has deemed the best kids program yet! (She's 8 and has been on 12 cruises!)
As for Key West, I suggest not booking a shore excursion and instead exploring the town by foot. There are a number of fun museums which feature treasures retrieved from sunken ships off of Key West, lots of great shops and restaurants, and a small aquarium. We also visited Nancy Forrester's Secret Garden which is a bit off the beaten track (but within walking distance of the pier) and features exotic macaws and parrots. Very neat since some of the animals even do tricks!
Are you also calling in Jamaica? If so, your teens will love going on a shore excursion to Dunns River Falls. My daughter was able to climb up the falls when she was 7 so I'm sure your teens will have no problem!
Luisa Frey Gaynor