Pavillon de l'Arsenal

Built in 1879 as a museum, this magnificent glass-roofed building with arched wrought-iron girders wasn't actually used as one until over a century later, when it opened as a centre for Parisian urbanism and architecture. Exhibitions (30 per year) showcase the city's past, present and future. Interpretative information is in French but it's fascinating for anyone with an interest in the evolution of Paris. There's a small but excellent architectural bookshop on the ground floor.