Sainte Chapelle. I'm not totally sure words can describe what it was like to walk in there. I had avoided the place while I lived in Paris: I was cheap, it annoyed me that you had to pay to go in, and the line was always absurd. But let me tell you, it was worth the 8€! We got there about 30 minutes before it closed, which is more than enough time to go through it and there was no line that late (5:30pm I think?).
I don't know what I was expecting, but even with one side under renovation construction, walking in to that main chapel is completely awe inspiring. All that stained glass!! All that colour! You are just completely surrounded by it. Really. Beyond words. The place is small but so, so impressive. We were early enough that there was still sun coming in through the windows, and late enough that they turned on the chandeliers, which gave it a neat look. For pictures anyway. I was snapping away like a mad woman.