Grande-Terre is itself relatively small and explorable in a few days, but its position makes several nearby islands accessible for day trips.
La Désirade is the closest: a long, flat island visible from Pointe des Châteaux, reachable by a short ferry from Saint-François. It is quiet, largely undeveloped, and has some of the least-visited beaches in Guadeloupe. The crossing takes about 45 minutes and ferries run daily.
Marie-Galante is a larger island about 30 km south of Grande-Terre, known for its rum distilleries (three functioning ones), unspoiled beaches, and a slower pace than anywhere on the main island. The ferry from Pointe-à-Pitre takes roughly an hour. A day is enough to visit a distillery, eat well, and swim at one of the excellent beaches. Some visitors find it more relaxing than anywhere on Grande-Terre itself.
Les Saintes (Îles des Saintes) is a small archipelago south of Basse-Terre, not Grande-Terre, but reachable via ferries that also call at Pointe-à-Pitre. The main village of Terre-de-Haut is a popular half-day or day trip destination with a pretty harbour, Fort Napoléon, and calm swimming beaches. The island is small enough that a single day covers it comfortably.
Basse-Terre is connected to Grande-Terre by a road bridge and does not require a boat. It is worth a day or two for the rainforest interior of Parc National de la Guadeloupe, the Soufrière volcano, and the waterfalls — a complete contrast to the flat, dry landscape of Grande-Terre.