Don't have any particular advice for the islands you're visiting, but wanted to share an opinion:
My wife and I have done both (booked excursions and tooled around on our own). We usually pick one island (during the course of our week) that sounds really interesting and book a shore excursion through the cruise line. For example, in March we took an island overview tour of St. Kitts and last year we did the same on Martinique. We saw parts of the island(s) that we wouldn't have otherwise.
Then, for the rest of the cruise we're on our own. Usually it's not very expensive to hire a taxi to take you to a beach. Some of my favorite memories of our cruise trips are days at the beach. We also like to just snoop around whatever is near the ship.
We were in Aruba last March. It was REALLY hot and it was the last stop - we basically decided to walk over to some of the shops, have an ice cream and come back to the ship for a nap. We talked to other folks that really enjoyed a 4-wheeling expedition and some others enjoyed a horseback ride on the beach.
That's the beauty of a cruise - you can do whatever you want, whenever you want to!
Anway, hope you have a wonderful cruise!
mhrxman
..Joanne