One of the great inevitables of Europe is the plethora of churches. Large cities have their landmark churches, tiny towns have their forgotten churches, and there are too many to count in between. I've always loved churches. I'm not Catholic, but I always find a moment of peace while walking along the wooden pews and gazing at the stained glass.
My favorite is when you can see light reflected in different colors on a somber, gray surface.
The Cathedral of St. Denis became famous when the kings of France chose it as their designated burial spot. There are tombs throughout the cathedral commemorating the centuries of French kings. The city itself is rich with history and has grown in its own way right alongside Paris.