These are 4 must see sites on a tour to Paris:

Musée Carnavalet
Métro: St-Paul or Chemin Vert.  Bus: 29, 69, 76, 96.
Museum Pass:  Not needed – no admission charge. Closed:  Monday.
A worthwhile stop while meandering through the Marais, it is the museum of the history of Paris.  The collection here is quite varied.  For instance, there are dioramas of the long-gone Bastille, paintings of the construction of the Statue of Liberty, and personal effects of Marie Antoinette. I enjoy viewing the uniquely crafted Parisian shopkeeper signs.   1-1 ½ hours.  Website:  www.carnavalet.paris.fr.

Musée des Arts et Métiers
Métro: Arts et Metiers. Bus: 38, 47, 75.
On the Museum Pass? Yes.  Closed: Mondays.  Open late Thursday.
Paris is well-known for its contributions to art, cuisine, and humanities. But let’s not dismiss a rich legacy in the fields of science, mathematics, engineering, and technology.   The 80,000 item collection of historical artifacts here is impressive.  For instance there are sound recording devices by Edison and a French airplane pre-dating the Wright Brothers along with early automobiles, TV’s, computers, engineering and technology marvels.  It is one of my favorite museums in Paris.    About 2 hours.  You would think they would have a better website though:  www.arts-et-metiers.net.

Musée National du Moyen Âge-Thermes et Hôtel de Cluny (Cluny Museum)
Métro:  Cluny La Sorbonne, Saint Michel, or Odéon.  Bus: 21, 27, 38, 85, 87.
On the Museum Pass? Yes.  Closed: Tuesday.
This museum of medieval art is located in the heart of the Latin Quarter at the intersection of boulevards Saint-Germain and Saint-Michel.  It is on the site of ancient Gallo-Roman baths.  Visitors get up close to unforgettable works of art from the Middle Ages like stained glass from Sainte Chapelle or the famous tapestry of The Lady and the Unicorn.  This is one of my favorite hidden gems.  Have a take-out lunch in the beautiful Roman gardens outside.  I think it is one of the most under-rated museums in Paris.  1-2 hours.  Website: www.musee-moyenage.fr.

Musée Marmottan
Métro: Muette.  Bus: 32.
On the Museum Pass? No – admission is €10.  Closed:  Monday.  Open late Thursday (7:30 p.m.). 
A museum containing some fabulous works of Impressionists, especially Monet, featuring his early works.  One of the best kept secrets in Paris.  A comprehensive collection of Monet’s paintings are displayed in a relaxing atmosphere, quite different from what is found at the Orsay.  Impressionist art lovers should not miss this museum.  A little bit out of the way.  No photos.  About 90 minutes to 2 hours. Website: www.marmottan.fr.