I don't know where you're staying or if you are going to risk life an limb renting a car, but two places you might want to visit are the Mamiku Gardens and the Diamond Botanical Gardens. Nick's blog:
A few butterflies from the island of St. Lucia, in the Caribbean (thanks for the IDs Nick):
Cloudless Sulfur Phoebis sennae
Stinky Leaf Wing Historis odius
The butterfly above was high up on a palm and too distant for a good shot, but I tried. It was a very large butterfly.
White-tipped Black Moth Melanchroia chephise
Little Yellow Eurema lisa
Julia Heliconia Dryas julia
Gulf Fritillary Agraulis vanilla
Common Long-tailed Skipper Urbanus proteus
Caribbean Buckeye Junonia evarete
Crimson Spectacled Moth Utetheisa pulchella
Baracoa or Fiery Skipper
Diamond Falls in the Diamond Botanical Gardens
The Pitons on a hazy day.
The Pitons from the sea.
A view from Mamiku Gardens
View from Pigeon Island