You’ve seen pictures of Ste. Chappelle, which appear often in travel literature and books about Paris, but no photo can do it justice.
Ste. Chappelle is maybe 200 yards west of Notre Dame. It is a compact space in very high demand, so you will want to book a timed ticket well ahead of your visit. Due to heightened security, even with a timed ticket you may have to wait (ours was about 45 minutes past the set time). Unfortunately, the complaints on this site about the poor line management are true - make sure you have a timed ticket, but anticipate a wait.
They take the excellent Paris Museum Pass.
It’s worth the wait. The core of Ste Chappelle is a large chapel with stained windows from the 1500s. The detailed windows have countless Biblical scenes from the Old and New Testaments. They have guides to help you find them and it is fun looking for things like the ark, Adam & Eve, David & Goliath, etc.
Look closely at the large rosette window at the back, which has scenes from the Book of Revelation, including multi-headed beasts and dragons.
A few notes: The chapel is on the second level of the building, and access may be difficult for those with mobility limitations. Some of the windows are also undergoing renovation, so portions are covered by scaffolding.
This is one of the most beautiful sites in Paris. It’s more than worth a visit even if the wait is longer than it should be.