Could the following question be reopened? I'd like to turn a comment I posted there into an answer:
Which sights are open late in Paris?
The question was already edited to turn it from just “What to do on just one evening in Paris?” to add “Which sights are open late in Paris?”, to which I’d like to post the following as an answer.
Proposed Answer
I’d suggest taking a look at the website of the Paris Convention and Visitors Bureau, which has a page titled “Places to visit at night in Paris”. It lists a number of places (mostly museums, some galleries) that have late openings. The page is unfortunately organized by place rather than by what your options are on a particular day of the week, but the “fact sheet” for every place does clearly list “late opening” on the “prices and times” section.
Here are a few suggestions taken from that page, but organized by day of the week (non-exhaustive list):
Monday: Musée Jacquemart-André (until 9pm, only during temporary exhibitions).
Tuesday: Jeu de Paume Paris (until 9pm).
Wednesday: Musée du Louvre (until 9.45pm).
Thursday: Musée des Arts et Métiers (until 9.30pm), Musée d'Orsay (until 9.45pm), Musée du Quai Branly (until 9pm).
Friday: Musée du Louvre (until 9.45pm), Musée du Quai Branly (until 9pm).
Saturday: Musée du Quai Branly (until 9pm), Musée Jacquemart-André (9pm, only during temporary exhibitions).
Obviously these times can change, so do check the original “fact sheet”. For most places it also lists until what time before closing time the ticket offices stay open, which would be useful to know.
