For use of Starwood points, your choices are limited to Puerto Rico, Grand Cayman, and St. John. Based on what you described I would probably recommend Puerto Rico. There is plenty to do, with El Yunque rain forest a short drive away. The beach is decent if not spectacular.
St. John is a lovely island with gorgeous beaches and wonderful snorkeling. There is not a lot else to do, but you could boat around to other islands and explore them.