Hi, Sara!
Congratulations on your first cruise aboard the
Disney Magic to the Western Caribbean! For me and my family, it's easier (and certainly less worrisome) to book
Port Adventures through Disney Cruise Line. I can get anxious about the details...especially as it relates to getting back to the ship on time. Therefore, it is worth it to me to have Disney do my worrying for me. From the transportation to the fun and back to the ship, I don't have to keep checking my watch or figuring out our plan while we're in port.
For Western Caribbean Port Adventures, you're really going to have to roll up your sleeves and do a little research. First go to the Port Adventures Page on DisneyCruise.com and narrow down your choices according to your children's ages. As far as comparing Cozumel and Grand Cayman Port Adventures, there are going to be several similarities as Western Caribbean port cities. However, the main difference to me is the history of the Mayan ruins available in Cozumel as opposed to Grand Cayman. Therefore, I would focus on surf and sea on the island and experience the culture of Mexico in the other port.
For Key West, I would consider a relaxing walking tour of the port for a few hours unless you see a Port Adventure that really peaks your attention, and for Castaway Cay, get there fast and take it slow! That's your day to take it easy! I hope this helps you plan, and let me know if you have more questions. I'd be happy to help you more.
Have a wonderful day and a magical vacation. Bon voyage!
Andi