Remember that the Journee du Patrimoine is next weekend (15 - 16 September).
Government and public buildings and facilities that are not normally open to the public will be open during the weekend.
The programme can be found at http://www.journeesdupatrimoine.culture.fr/
Someone has posted a personal selection on Tripadvisor, which is well worth looking at.
If you are in Paris for that weekend, skip the Louvre and the Eiffel Tower, and visit somewhere a little different, that most people don't get to see (of course the Journee du Patrimoine is not just restricted to Paris - whatever French city you are in for that weekend is likely to have a programme!)
For ideas of where to go for your day trip from Paris, see www.beyond-paris.com