Been to Paris - be prepared for LOTS of tourists at this time of year, which means long lines and waiting at the most popular tourists points.
I personally would highly recommend the Rodin Musuem. And check out Napoleon's grave - it's a BIG tomb-like thing in the middle of the city - just remember how big he was and then notice how big the tomb is!
In all of them though don't forget to just wander around enjoying the city, not on any tourist schedule (if you can), write postcards in a cafe, etc. Those are the moments when you really get a feel for a city.