you are right on tract, resist the urge to try to cram in as much as you can into your day. For me 2 excursions per day is too much, i just do 1 excursion per island, usually in the morning, so that leaves my afternoons free to just wander around, shop whatever. Some of my fav excursions:
Nassau - take the tour of the "Dig" the aquarium at Atlantis Resort, its amazing, do it on your own, no need to book a ships tour, just grab a cab -either land or water and head over to Paradise Island.
St.Thomas -if your cruise offers book the Champagne Catamaran Sailway. this is a great tour -you get beautiful sail, beautiful beach & snorkeling.
St. Maarten - America's Cup Regatta or the Golden Eagle Catamaran Sail.