we sailed on the Celebrity Summit in January for a southern caribbean cruise. Grenada -- Definitely go to Grand Anse Beach -- it's beautiful. There's a little beach bar called Garfields that has cheap drinks. The water and sand is perfect there.
Dominica --if you are there at the right time you can book whale watching tours. We went on one and although we didn't see a single whale we did see close to 300 dolphins that raced our boat. It was a blast!
Barbados -- take a local bus up to Mullin's Beach Bar. It's great! Very nice restaurant -- great food. Tell Eugene the bartender hi! You can rent beach chairs and umbrellas for about $10. There's a floating dock you can swim out to. Jet skis can be rented nearby. The bus trip is cheap (I think it was about $1.50pp) and its a hoot. Get out of Bridgetown immediately, kind of dumpy.
I loved the Southern Caribbean -- probably my favorite so far! We also went to St. Marten's and Tobago -- great beaches!