Nuit Blanche!

An annual event that takes place all over Paris consisting of art expos and galleries staying open all night. Special events are held all over the city for the evening. Something for everyone. I would recommend having a loose plan but taking some time to wander and discover things around a particular neighborhood. 

I wish I could tell you why the church is lit up, but the line was down the street, and I couldn't get in! Be prepared for that sort of thing as well.