People usually go to the Louvre to see the Mona Lisa and other art from the Masters. That's what we did the first time too. What happens when you luckily get in for free two hours before closing on a Saturday night? You go around and see whatever you can!
Michelangelo Pistoletto decorated the Louvre's pyramid for the first time in its history with a sign against capitalism. Ironically it looks like the All Seeing Eye on the almighty one dollar bill. He has hidden several pieces all over the museum and has a couple more obvious installations each carrying a message or metaphor.
The Louvre is huge and you could/should spend at least one whole day here if you want to see everything. We snapped photos of all the interesting stuff we missed our first visit. Unfortunately, I only recognize Venus de Milo from this interesting bunch of photos.
Feel free to chime in on the comments if you recognize anything else!