Have you checked out the suggestions on our sister site "portreviews.com". That's a good place to start to get ideas of things to do. If you are looking for a non-touristy but great beach experience in Aruba try the Havana Beach Club. Located just outside the downtown district, right before the airport, it is a local spot (easily within walking distance from the port) but they have a nice (but small beach),beach chairs, bar, restaurant - everything you want to get away from the "typical tourist haunts".
As for St. Lucia - my favorite beach is Anse Chastenet (a gorgeous black sand beach).