This is a very interesting small museum inside the Tuileries garden (garden of the Louvre). It is world-famous for the big canvases of water lilies that Monet worked on for the last 10 years of his life, and just for that it is worth visiting. But there is a big bonus attached to it: it's the Guillaume/Walter collection. Not only will you see paintings by Renoir (many), Cézanne (many) and Monet (one painting), you will see avant-garde paintings of Picasso, Matisse, Modigliani, Douanier Rousseau, Utrillo, Soutine and many others.
A funny thing about the Orangerie, I can’t decide if what's the most extraordinary is the paintings themselves or the incredible story of how they ended up there. You'll tell me!
The visit of the Orangerie can be done in one hour (if we rush things just a tad) and it can easily be visited before or after Louvre or Orsay.