One of the many reasons I plan my trips to Paris in October is so that I can go to Nuit Blanche, which is always an interesting night. Start Saturday night and wander around the city, stumbling across fantastic installations, until the next morning.
It is really a cool experience, especially when you start getting into 3 or 4am and the amount of people have dwindled drastically. The crowds are gone, the rowdy people have probably passed out by then, and all that is left are a few groups of people that have hung in there, sitting around in an installation. Laying in Parc de Belleville listening to the symphony that was composed for the park with speakers scattered over the entire park. Or back at République for the first metro home, and the square that you first came across when it was filled with people -yelling, screaming, drunk, being idiots, and climbing to the top of the statue- is now almost empty, hushed, and still completely covered in the fog from the installation.