Your first essential stop has got to be the magnificent stretch of the sandy Orient Bay facing the Atlantic Ocean on Saint Martin's northeastern coast. There, far from the tranquillity of the Caribbean Sea, giant waves smash against the rocks. Split into two, the beach is nudist for the most part.
After that, why not treat yourself to a stop on the smallest of Saint Martin's 37 beaches, well hidden in a cove and protected from view: Lovers Beach. Locals will confirm that it owes its poetic nickname to its size, so small that it can only accommodate two people at a time, and for just long enough to steal a kiss. Next is Happy Bay, which well deserves its name. With your toes in this stretch of soft, fine white sand, you will likely not want to be anywhere else in the world.
Other marvels await you, especially on the side of Creole Rock near Grand-Case Bay. Here, it is time to don flippers, mask, and snorkel and dive into the warm crystal waters among shoals of butterflyfish. If you are lucky, you might see stingrays and turtles (but if you really want to swim with turtles, it is best to go to the tiny island of Tintamarre, a wonderfully wild and protected site that you will not want to miss).