We may consider snorkeling. Here is a snorkeling site that shows up in several web pages:
"For the best snorkeling on St. Lucia, head to the waters in front of Anse Chastanet beach. The Anse Chastanet Reef is the perfect haven for snorkelers because it is buoyed off for snorkeling safety, so no boats are allowed in the area. There is a plateau on the reef that reaches from 5 to 25 feet deep, and at its end, falls away from 20 to 140 feet into a coral wall full of amazing sea life. Photographers favor the reef as one of the most picturesque sites in St. Lucia."
I will post some pictures of Anse Chastanet beach. As I understand it, one of the best features of the beach is the view of the Pitons.
2. Sailing:
[I will add my findings on this option.]
3. Exploring:
St. Lucia, like Dominica, is a natural wonderland--a sort of Garden of Eden. It includes beautiful rain forests, waterfalls, and trails throughout. [I will add my findings on this option as well.]