Very close to Notre Dame is the Holy Chapel, which is a chapel with Gothic stained glass windows from the 13th century! This alone would justify a visit, but there is more!
It was built by Louis IX in the 1240s to house relics from the Holy Land considered to be the Crown of Thorns and part of the True Cross. He built the chapel to house the treasure. This small Gothic chapel is one of the most inspiring visual experiences in Paris. This is largely due to its stained glass windows that essentially surround the entire upper floor. The stained glass windows tell the story of mankind (or at least the history of mankind that was known until 1248). King Louis IX is the only holy king of France.
The chapel itself is now surrounded by the Courthouse on Île de la Cité near Notre-Dame. It has two “floors”, the first at ground level being rather dark and close, the second having high radiating windows and a small balcony.
10 Boulevard du Palais, 75001 Paris