Cité de l'Architecture Museum
If you are interested in Architecture, you must visit the "Cité de l'Architecture" in Paris which is a complex housing several museums related to the subject.
Their collection of monumental architecture casts will take you back several centuries, from to the middle ages in the 1100's and up to the 1700's... You can also admire models of churches that have long disapeared.
As you know, the Louvre and Orsay museums draw the largest crowds of visitors, so the great thing about this place is you will get to appreciate everything at your own pace without having to elbow your way to a piece of art...