If you are a fan of modern architecture, you must visit Villa Savoye. Villa Savoye is one of Le Corbusier's architectural masterpieces; a single family, extremely modern (even by today's standards) 1920s house now open for you to mosey around.
The day I went, the house was almost empty. There were only three or four other people there, quite different than Versailles. Villa Savoye, at least when it is empty, is immensely relaxing. In some rooms there is antique furniture by Le Corbuisier that you can actually sit in! How often do you get to sit in the furniture at a museum house? My friend fell asleep in one especially comfortable lounge chair. The house felt so welcoming, and might even change some people's minds about modern architecture being cold and uninviting. Villa Savoye is only a train and bus ride away from Paris, and it is worth every minute.