We spent one day and night in Chartres on our way back to Paris from the coast. The main attraction here is most definitely the cathedral which can be seen from a considerable distance away.
There was a special pilgrimage on the day we visited so we had limited time inside but it's certainly on a scale with Amiens and Reims among the great French cathedrals we've visited.
We stayed a few blocks away from the Cathedral. Chartres is a very compact town and everything seems to be in easy walking distance of the center. Restaurant and shop hours are pretty strictly observed here so if you wander out at the wrong time of day you'll find that nothing is open.
The chancel of Chartres cathedral was in the process of being extensively cleaned and restored to a brilliant white color (from the rather drab grey you normally see).
The cathedral sits up above the Eure river on a hill. The Eure isn't particularly impressive here, in fact you can easily miss it (it's tucked in front of the timbered houses in this picture).
The main reason we spent the night in Chartres was to see the Lumieres which is a projection light show that involves 23 different buildings around town. Some of the projections are just static displays on a wall, others are extremely elaborate. The most impressive are those on the Palais de Justice and the assorted displays on the cathedral itself.
The front of the cathedral features a long cycle that documents the major historical periods and events in Chartres history set to music. This lasts about fifteen minutes and we watched it both at the beginning and the end of our Lumieres circuit because it was so impressive.
Lumieres starts... when it gets dark. In June, that was close to 11:00 PM. It ends around 1 AM and we actually had to rush a bit to get through all the sites in two hours. It would actually be a little easier to do this further from the solstice.
Along with the front of the cathedral there are light shows on both sides and in the back of the cathedral. This lighting does a great job of turning the apostle statues on the north side into ghostly figurines.
Eglise Sainte Foy has an interactive display. In this case we dialed up the ocean background and placed the animated origami sea life figures onto different parts of the church (this is surprisingly entertaining).
Saint Aignan church has fabulous stained glass windows which are augmented at Lumieres by projections onto the walls around them and even the flying buttresses that support the walls.
There are also several spots along the Eure that are lit up including four bridges. This one was nearly static, but every minute or so the eyes would blink for a rather creepy effect. If you're planning to walk the Lumieres circuit in Chartres, there are small blue discs embedded into the sidewalks that glow faintly at night so you can follow the path. These are not particularly easy to find and follow so having the lumieres map (and maybe a phone) also helps.