Here is some info I can share with you about your ports....an island tour is always a great way to get the lay of the land, and see alot...and you can always beach it everywhere!
St. Thomas If you like to snorkel, you could take the ferry over to St John, where it's amazing. Take the tram to the top of the mountain, shop the flea market by the psot office...
Dominica has mostly eco friendly tours, if this is not your cuppa tea, you can save $$ here..no shopping to speak of either.
Barbados Visit Harrisson's Caves, a spectacular cave system, you get around on trams, totally amazing!
St. Lucia have not been
St. Kitts have not been
St. Marteen.. the America's Cup tour is $$ but I know people who have done it and loved it! You can save on other ports, and splurge here. Something to do once, if it's up your alley