Just look at this picture (especially since it's the only one on this page); it speaks for itself!
This Holy Chapel, built in the 13th Century by king Saint-Louis (Louis the 9th) as a shrine for the Holy Crown of Thorns of Christ, was recently beautifully restored (stained glass and outside/inside walls). You'll find 6660 square feet of some of the most beautiful stained glass you've ever seen! (most from the 13th century, more than 2/3 are original, and the others are difficult to differentiate).
If you go to one of the concerts in the evening, you will savour the music while watching the stained glass becoming more and more purple (in the Middle Ages, people were referring to wine with "the color of the glass of the Sainte Chapelle"...).
I could tell you quite a few things about the Sainte Chapelle but you can't use me for EVERYTHING (not that I would forbid you to though). You could easily go there on your own (especially if you go to a concert). Otherwise allow one hour for the visit.
And if you've been to the Sainte Chapelle before and you loved the stainglass there, you HAVE to go to Chartres or Bourges or, even better, both as they are both equally stunning and complimentary.
Chartres can be done during a half-day tour, for Bourges you need a two-day tour minimum as it's 150 miles away from Paris.