Martinique is compact enough to explore fully in a day from any base. The most popular excursion by far is Les Fonds Blancs, the area of shallow sandbars off the east coast between Le François and Le Vauclin. Boat trips run from the Baie du Simon most mornings, giving you a few hours wading in waist-deep turquoise water in the middle of the open sea, including a stop at the Baignoire de Joséphine sandbar. A Creole lunch on board is usually included.
The southern beaches at Sainte-Anne and the offshore Diamond Rock make for easy half-day trips from Fort-de-France. The Caravelle Peninsula on the northeast coast — a headland protected as part of the Regional Nature Park — rewards a longer outing, with a clifftop walk to the ruins of Château Dubuc taking around two hours.