I've been trying to get out to see as many museums in Paris as I can.  One of the oddest is the Musee des Arts & Metiers.  It is billed as a "depot of new and useful inventions" and it houses all sorts of random, forgotten and downright weird artifacts.  19th century flying machines. Early cameras.  Zoetropes. Magic lanterns.  Focault's pendulum.  Cotton gins.  Steam engines.  

It's like the attic of France.  If you are a fan of steampunk you'll love it.  If not, you still might be carried away by the space, since part of the museum is set in the old priory of Saint-Martin-des-Champs.  They've taken this old church and added a very cool modern superstructure to house exhibits.  It's like a movie set.  I think this is the only place in the world you can see a bi-plane flying through a cathedral.