so much depends upon what you want to do, what your likes and dislikes are. A great place to start looking for excursions would be Carnival's web site, i would also go to the library and check out either Frommer's or Fodor's travel guides to each port.- you can also go to their websites for some really good information.
Depending upon when you arrive and what time the ship leaves in San Juan will determine if you have time to do any type of excursion. Just know when planning your trips that you MUST be on board the ship AT LEAST 45 minutes to an hour prior to sailing time.
A great resourse for the US Virgin Islands it their website: www.vinow.com