Barbados - arrange your own tour with a taxi driver
Aruba - island tour or go to a beach (the beaches here are very nice) or both
Curacao - we just wandered around downtown - got some great photos of the pontoon bridge and the Seaquarium is supposed to be good
Venezuela - I have only been to Margaritia Island - if that is where your ship is going I recommend an island tour
Grenada - tour to National Park (lovely scenery and monkeys) and a trip to Grand Anse Beach
Curacao - great to just wander around downtown. The Seaquarim and beach are nice, we did a submersible tour that was really fun.
Grenada - do a spice plantation and rain forest tour, very interesting, gorgeous scenery.
Venezula has some beautiful beaches
It's been ten years since I was in Barbados, but we arranged a price and destinations with a taxi driver. We went to Signal Hill (brief stop to see the view) then on to Bathsheba on the east side of the island. There are big waves that crash into large bolders that have eroded to look like giant mushrooms. Not far from there we saw an old church and a beautiful botanical gardens. This gardens was hardly crowed at all since most of the cruise tours take passengers to the other gardens that is near Harrison's cave. We were very happy with the tour and the driver who gave us history and other info during the whole trip. We still had time to shop in Bridgetown before it was time to board.