I know that people will have a TON of Paris suggestions, but we strongly encourage folks to add the following to their sightseeing stops:
Musee d\'Orsay -- a wonderful collection of impressionist art in the former Gare d'Orsay train station; a must-see on each of our trips to Paris; we prefer this to the Louvre.
Sainte-Chapelle -- possibly the most beautiful stained glass windows we have ever seen (close to Notre Dame).
There's never enough time to experience Paris.
