The Musée de l'Orangerie is small by Paris standards, but it contains one of the city's great art experiences: Monet's Water Lilies installed in two oval rooms designed for slow looking. The paintings wrap around you in bands of water, sky, willow, and reflected light, turning a museum visit into something closer to sitting inside a garden pond.
Downstairs, the Walter-Guillaume collection adds Cézanne, Renoir, Matisse, Modigliani, Picasso, Soutine, and others, giving the museum more range than its size suggests. It is beside the Tuileries and Place de la Concorde, so it pairs naturally with the Louvre or Musée d'Orsay, but it deserves its own hour rather than being treated as a spare-time add-on.