Île de Cézembre is a small island about 4km west of Saint-Malo, reachable by boat from the port during summer months. The island is uninhabited — the German fortifications from the Second World War are still visible, along with the bomb craters from the Allied assault. There is a beach on the south side that is among the cleanest on this stretch of coast.
Boat trips to Cézembre run from the Grand Porte quay in summer, taking about 20 minutes each way. The island has no facilities beyond a basic bar in season; bring what you need. The ruins of the German bunkers make for interesting exploring, and the views back toward Saint-Malo and across to the Normandy coast are excellent. On a clear day you can see the outline of Cap Fréhel to the west.