Sorry to be a downer, but there are three (and almost four) unexpected closures that have put a slight damper on a multi-week trip to France:
It's been passingly mentioned in other posts, but the Helbronner gondola (the 40 minute ride to Italy from Mont Blanc) is closed for the entire 2018 season due to an accident in January 2018 (read more here).
Aiguille du Midi
While we there (Aug 2018)--but luckily after we had made the trip twice--the cables had some kind of problem and they shut the gondola down for two weeks without warning. If you were planning on making a special trip to Chamonix for this experience in particular, you might want to call ahead and confirm it's running...and signs don't look good for the fall (read more here).
The vast majority of the Cluny is closed for renovations until the end 2020. You can still see the Unicorn tapestries and the fee is reduced to 4E, but if you were looking forward to the rest of the museum, no go.
des Egouts is closed for renovations until 2020. No access at all.
Sorry to be the bearer of bad news!