Personally, I wouldn't do any organized excursion with that amount of time.

The whole island is an experience. I might rent a car or pay a cabbie who will wait or return for the day to drop you off at various places such as Pedro's at Orient beach and stroll have lunch and enjoy the beach, then go to Grand case and then Marigot and mullet beach and maho beach Simpson bay and phillipsburg. Then you'll know where you want to stay and go on your most probable next trip.