The Strasbourg cathedral may not be the biggest or the most famous cathedral, but it's the only one that consistently takes my breath away every time I see it. It's tower is visible from a distance as you approach the city, but, because the buildings of the medieval city cluster closely around it, you don't see it again until you step out of a narrow path and discover it towering above you. It's awe-inspiring.
There happened to be a children's Christmas concert when we went into the cathedral. They sang christmas songs and re-enacted the Nativity story.
Oh, and there's a giant old clock in the corner with creepy figures that move when the bells chime.