It's always good to check out what's happening in Fondation Cartier. They have lots of really good exhibitions of modern art, and the gallery itself is very beautiful. Perfect for seeing works you already know as well as discovering new artists and inspirations.
You enter La Fidelity and will find a very classic Parisian Brasserie, probably almost empty. Maybe one or two people at the bar. In the middle of this place you will find the stairs that go down to the cave. Have fun!