Just a heads up, I was in Paris 2 Christmases ago and had to stop into Lafayette and Printemps for some shopping. I just figured I'd head to the roof and kill some time, but this was not a unique idea. As I ascended higher and higher the crowds got even thicker until finally, just before reaching the roof, we had to wait in line for some minutes because it was too crowded up above.
Oh and that year there was a giant astronaut Santa in the atrium of the Lafayette, for some reason.