I recommend the Land and Sea Tour of St. Lucia, although be prepared for a very full day; it's a long but memorable tour. If you get Martina Jules as your tourguide, all the better. She was wonderful: fun, funny: she sang songs, taught us a few key words useful for your time in St. Lucia, and tossed out a few riddles. The answer to one of them is Khaki. (All you'll have to do is pay attention and give her that answer to the appropriate riddle!) The tour was very comprehensive: we saw a collapsed volcano, a lovely garden, a waterfall, a scenic view of the Pitons, and a very refreshing power catamaran ride to a nice spot where we were able to swim (to a beach if you wanted). St. Lucia stands out from all the other ports of call due to that tour. You really feel like you took in the island's beauty and wonder. If you go on this tour, and get Martina as your tour guide, please, please tell her Paul from Portland, Maine is hoping she gets a free hour to drop him a line (she has my address). It's preferable to do the land portion of the tour first, as you can unwind from that on the catamaran.
I also did the Barbados Harrison's Cave and Flower Forest tour which was good. You have to be careful in the cave (you're on an open "car" with about 15 people): don't let your hardhat fall off and out of the car. I thoroughly enjoyed the Flower Forest as you are on your own for about 45 minutes and can take it at your own pace.
There's a beach break excursion in Antigua, but my friend and I didn't want to be there for 5 hours, so we got a taxi for $10 bucks each way; if there are more of you, all the better: if there are four, for example, it's only $2.50 per person, each way (plus tip). The beach is wonderful, scenic, with shade, rest rooms, etc.
Have a great time, and be sure and let me know if you meet Martina Jules!
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