Everyone needs to take a day and see Chartres and its Cathedral, about 50 miles south and west of Paris, city centre. Take the metro to Gare Montparnasse, which is just south (and next) to the tall skyscraper that dominates the Paris skyline. There you will find the connection to Chartres (leaves top floor). Trains leave often from the Montparnasse train station (about every hour); but, be sure to know the departure schedule of last train back to Paris, 'cause there is no other convenient way back. Get a round trip ticket; it takes about an hour on the train. Cost-about 12 to 13 Euros each way.
The Cathedral became the most important building in the town of Chartres. It was the center of the economy, a most famous landmark and the focal point of almost every activity that would be today provided by civic authorities. Today's structure sits on a site occupied by previous churches and fronts what was once a Roman forum. A center for pilgrimage, since at least 876 the Cathedral's site has held a tunic that was said to have belonged to the Blessed Virgin Mary, the Sancta Camisia, presented to the church by Charles the Bald. http://sacred-destinations.com/france/chartres-cathedral.htm The abbey church of St-Pierre, dating chiefly from the 13th century, contains, besides some fine stained glass, twelve representations of the apostles in enamel, created about 1547 by Léonard Limosin. Of the other churches of Chartres, also noteworthy are St-Aignan (13th, 16th and 17th centuries) and St-Martin-au-Val (12th century). See also la Musée des Beaux-Arts, the fine arts museum (located next to the Cathedral of Chartres) housed in the former episcopal palace. http://en.wikipedia.org/wiki/Chartres
Original Chartres was built on the left bank of the river Eure, SW of Paris about 30 miles. Chartres was one of the principal towns of the celtic tribe called the Carnutes The Roman name was based on the river Autura (Eure), and afterwards became civetas Carnutum, which has evolved into the current name. It has changed hands and been besieged several times. It was taken in 1591 by Henri of Navarre, who was crowned there three years afterwards as Henry IV. Through all the fires and wars its windows have survived, telling their bible stories and other tales of France -- and the giant rose window still illuminates the entry way.
As a short side-trip, when you return from Chartres, see the cemetery near to (northeast of the Montparnasse station in Paris. Exit to one of the north-eastern exits of the Gare (or follow underground towards Metro Line 4 and use a NW street exit), or take Metro line 6 (Direction Nation) and go one stop to Edgar Quinet. Your looking for the street (rue Edgar Quinet), which runs from the Tour Montparnasse, along the northern edge of the Cimetière du Montparnasse, towards the Raspail Metro stop (lines 4 & 6) -- where incidently the remains of Dr. François Vincent Raspail now rest.
Many of the famous as well as obscure rest there, heroes of the 1870' and the Great War and the Resistance in WW II. Interesting places of internment. See the final earthly destination for Serge Gainsbourg, Susan Sontag, Jean-Paul Sartre, Samuel Beckett, Simone de Beauvoir, Baudelaire, Jean Béraud, Brancusi, Brassaï (the eye of Paris), Duras, Saint Jean Seberg, Camille Saint-Saens (Printemps Qui Commence), Ionesco, Mieroslawski and all the LeClercs. Dreyfus is buried in Montparnasse cemetery. Ironically his grave is not far from the wife of Maréchal Phillipe Petain. Brassò (from Transylvania-his pseudonym meant from Brassò) died on July 7, 1984, in Beaulieu-sur-Mer. He was buried in Montparnasse cemetery in the heart of the Paris he had paid tribute to for more than half a century. You'll find a map in the Cimetière that shows where many of the popular sites are; but just walk around, in order to see much more.
The park that is in the middle of the street (rue Edgar Quinet) also hosts an upscale Art "Flea Market" (marché puces) -check schedule- I think it is Sunday morning all year long.