With your 21-year-old, I agree with Pat that the Maho area is the best for you this trip, and I also agree that Royal Islander La Plage is the best resort for you in that area. While I would never suggest going to SXM and not renting a car, based on the criteria you've described, I understand not wanting a car; and if you don't have a car, then Maho is the place to be.