At low tide, cross the beach to walk out via Porte des Bés to Île du Grand Bé, the rocky islet where the great St-Malo-born, 18th-century writer Chateaubriand is buried. About 100m beyond Grand Bé is the privately owned, Vauban-built, 17th-century Fort du Petit Bé. Once the tide rushes in, the causeway remains impassable for about six hours; check tide times with the tourist office so you don't get trapped on the island.
The owner runs 30-minute guided tours in French; leaflets are available in English.