No need to do any shoreexcursions - all it needs is a bit of research. If you use a browser and type in the words 'message and cruise and grenada' - of the results returned you need approx 3rd link down that begins with the word Grenada and contains the word 'boards'. Repeat for each island!
Many thanks for the information have started the research and hope to get some great ideas.
Wecruised QE2 last Christmas and can recommend the sugar train in St Kitts. It takes you all around the island - fantastic views and people so friendly. It's definately an experience.