The most remarkable Stained Glass windows in all of Paris, and its Not Notre Dame, but right near it. Sainte Chapelle

This breathtaking Gothic chapel, with its stunning stained glass windows, is one of the most exhilarating places in Paris. It was commissioned by Louis IX in the 1240s to house relics from the Holy Land, most notably the Crown of Thorns and part of the True Cross. Imposing and yet marvelously intricate, La Sainte-Chapelle is located on the Ile de la Cité near Notre Dame. It has two tiers: an intimate area at ground level with an awe-inspiring view, and a raised space with radiant tall windows and a small balcony. One of the great achievements of Gothic architecture, the Sainte-Chapelle boasts one of the most extensive displays of stained glass to be found anywhere. Bask in the light of bygone days and feel the warmth and wonder emanating from some of the most beautiful windows in the world.
Go early in the morning, during the Paris tourist season to get your tickets or you'l find long lines.
To recover the crowds after your visit, step across the square to the Les Deux Palais for a cafe.
Another way to avoid this, is to attend one of many chamber concerts held here in the evening.