Hi Tracy,
Welcome to cruise-chat. You might want to check out the reviews at our sister site:
http://www.caribbeanportreviews.com. There are some great ideas for each of those islands. What time will your ship be in port in San Juan? If you love beaches, I suggest Orient Beach in St. Maarten and Sapphire Beach in St. Thomas. My favorite activity in Nassau is a snorkel trip on the Flying Cloud catamaran to Rose Island.
Nancy