Giverny and Auvers-sur-Oise
Maintenon & Chartres
Otherwise, you'd never have guessed, but many half-day tours put together can make a full-day tour!
Here below are some other good combinations:
Ecouen + Chantilly (or Pierrefonds)
(it can be done in 8 hours if you don't spend too much time in the gardens at Chantilly).
Otherwise, almost anything is possible because YOU call the shots. For example we can make a full day by going to Giverny in the morning and to the Chartres cathedral in the afternoon (and that would make a very good tour too!).
Louvre Lens & Roubaix
Vezelay in Burgundy
Vaux-le-Vicomte and Fontainebleau
Versailles & Giverny
Believe it or not: this map is not accurate. It's just to give you an idea.