St Lucia = take a taxi tour to include the Volcano, Diamond Springs and Soufrier for a view of the Pitons. Allow lots of time as their roads are terrible.
St Kitts = if you can take a taxi tour to include the city, the ancient hyrogliphics, and the fortress. The view from a top of the fort is beautiful.
Barbados= get a taxi just outside the port shopping building at about $30 per couple. Have him bring you to Harrison's Caves (+ entry fee) and around the island.
3 to 4 hrs. for a tour is sufficient on most islands plus the tour of Harrison's which I think was about 1 1/2 hrs.