Museum Pass is a must. But, yeah, must also check the closing dates so it's not useless for the one you want to visit. (The Orsay and that little place called the Louvre are my personal faves)

Make sure to stop by the Opera Garnier. (eg: "The Paris Opera House" from Phantom) Its opulence rivals Versailles itself!

Actually, that's where I'll be staying when I go in Sept. It's the Hilton Arc de Triomphe