Hôtel de Cluny/Musée du Moyen-âge

The Hôtel de Cluny is an early 15th-century mansion on the Left Bank that now houses the Musée du Moyen-âge, appropriately enough. The hôtel is cheek-by-jowl with ruins of ancient Roman baths, some of whose spaces are incorporated in the museum. There's also a lovely public garden in the back.