The first thing to do in La Rochelle is walk the harbour. The Vieux Port is small, beautiful, and entirely walkable in a morning — start at the Grosse Horloge, follow the quay round to the Tour Saint-Nicolas, cross to the Tour de la Chaîne, and end at the more austere Tour de la Lanterne. These three medieval towers have guarded the harbour since the 14th century and you can climb all of them.
After the towers, walk inland through the arcaded streets — the Rue du Palais and Rue Chaudrier are the prettiest. The Hôtel de Ville is one of the finest Renaissance buildings in France. The Aquarium near the marina is one of the best in Europe and worth a half-day. South of the centre, the Bunker de la Rochelle and the Nazi submarine pens at La Pallice are reminders of the town's wartime role. The Musée Maritime tells the longer story of La Rochelle and the sea.