On a cruise stop in 2014
I did a 2-tank dive excursion booked privately with Scuba Steve's and liked it just fine. Given the estimated expenses of a dedicated dive trip to St. Lucia, that may well be the only diving I ever do there. Enjoyed it and no regrets. Back in 2006, on our honeymoon cruise did a rainforest hike there, and enjoyed that, too - very lush (didn't see wildlife, just vegetation, but a lot of it).
St. Croix has some good diving and I enjoyed it. It has other activities, but if there are no non-divers to entertain, I might go for diving there. It may seem a little 'sleepy' compared to some cruise ports, but is a beautiful island.
Antigua doesn't get much mention on ScubaBoard, but I have seen the topic brought up very sporadically. A cruise stop may not be the best way to blaze a trail. I'd do something else there.
Barbados gets mentioned in diving here once in awhile. Wife and I did a snorkel tour where they fed sea turtles on a catamaran back in 2006; rum punch was included and I thought 'the drunks' were hallucinating 'fairies' till I realized there were flying fish leaping out of the water and gliding in the distance. Might dive this one, simply because it's a place I might never go for a dedicated dive trip.
Never been to Grenada. It's well-spoken of on ScubaBoard, for topside and diving. I suggest reading some trip reports, and see what your excursion options are. If it appeals strongly and you think you want to go there for a dive trip, I'd do a topside excursion on the cruise. Enjoy something you otherwise might not.
Dominica - known for lush rain forest and some good diving. I'd be torn between a rainforest hike and diving. Here are some old threads I keep handy in case I ever get to go there:
Discussions of Dominica -
Dominica…everything I should know but don’t -
http://www.scubaboard.com/forums/gr...minica-everything-i-should-know-but-dont.html
I was surprised by how nice the diving was in Dominica. Excellent! -
http://www.scubaboard.com/forums/le...rised-how-nice-diving-dominica-excellent.html
Dominica Recommendations? -
http://www.scubaboard.com/forums/lesser-antilles/494111-dominica-recommendations.html
St. Kitts - when I see it mentioned in trip reports, it's generally because someone took the
Caribbean Explorer live-aboard, dove St. Kitts and Saba, and Saba was more dramatic and hogged the attention. Topside, St. Kitts is a pretty beautiful island. I've visited on a cruise stop, we hit Romney Manor and some 'great house,' and I paid for my daughter and I to pose with a pet monkey so I could get photos.
Either St. Lucia or Dominica could get you a nice rainforest hike, I think. I'd dive one or the other. Note: nobody who knows me would associate me with 'hiking,' but rainforests inspire me. Antigua I'd do something else. St. Croix I'd dive. Barbados I'd dive because I'd likely never get around to a dive trip there. Grenada I'd have to think about.