St.Thomas has 2 of my fav, the trip to St.Johns trunk bay or stay on St.t and do Meagans bay beach.
In Aruba we did the Bananna Bus to the natural bridge,had a blast, the guy who run this bus are great.
I'm a history buff, in PR do old san jaun/fortress
Oh that sounds like a really nice cruise. I've only been to St. Thomas and Grenada of the islands you've mentioned.. In St. Thomas I'd go to Trunk Bay, St. John, (see photo below). Take the ferry and go on your own if you can so that you can stay longer. If there is a Heaven on earth, it would be here.
On Grenada we did a wonderful sail/snorkel/beach excursion I believe it was with Widaldi which is usually offered through your ship. This one visited Mourne Rouge Beach, (awesome! secluded beach) and snorkeling stop at Flamingo Bay.
In St. Thomas, we tooka Sunny Liston tour (shopping, island tour, beach stop) that was very good. (We booked online ahead of time after reading good reviews). He meets you at the pier, drops you off for a couple hours of shopping, then picks you up and takes you for a tour of the island, then stops at a beach (we went to Sapphire beach, which was very nice). I have also heard Godfrey tours are excellent (kind of the same thing).
In Aruba we took an excellent snorkeling tour with the Jolly Pirates. They take you out on an authentic looking pirate ship, go to 2 or 3 snorkel spots, feed you lunch on the boat... It was a great time. You will just need to take a taxi to the meeting place.
In Grenada we took a spice island tour through Royal Caribbean that was about 4 hours long or so. It was too long. After riding on a bus that long, you will be ready to get off. We had a great tour guide, Henry, and saw some interesting places, but the narrow windy roads weren't that fun.
I am taking the same cruise a month later. You didnt say what you enjoy doing so it is hard to tell you what to do for excursions. Do you snorkel, hike or do you want to lay on the beach or be driven around on a bus? I am a diver so I have no trouble figuring out what to do. I would recommend taking a ride to Turtle Beach in St. Kitts-beautiful ride (catch a taxi on the pier) and there are monkeys at the beach.
Bonaire is known for its great diving and snorkling so if you want to do that, do it in Bonaire. Most of the beaches are not great-very corally. There is also a park with a flamingo preserve. Very quiet island.