Ditto SPF's question...
specifically:
1. day sailing:
c/o or otherwise, with snorkling stops, lunchtime beach stops. Last SXM visit in 90's we did a day sail to Prickly Pear., no snorkling but a fun ride.
2. St Barts & Anguilla day trips
3. horseback through the surf
4. Seadoo rentals