Hi madmanwc, welcome to Cruise-Chat! San Juan has some very interesting sites, but you can walk to most of them from the port. The beach would be a pretty good walk, but can be done.
In St. Thomas, there is Coki, Sapphire, and Megan's Bay. You'll have to take a cab there. If you want to go to Trunk Bay, you can catch the ferry from Red Hook, or Charlotte Amalie, then take a cab to Trunk Bay.
In St. Maarten, you can take the water taxi over to the beach or you can go to Orient Beach or Maho Beach by cab.
Have a wonderful time.