Hidden gems Paris?

I'm traveling to Paris from Saturday to Monday. I've been to Paris before and have seen all the major sights. This time it's a team trip, and we're going to see sights like the Louvre, the Sacré-Cœur, the Arc de Triomphe, Champs-Élysées, Eiffel Tower, Galeries Lafayette, and so on. Do you have any other tips?

Any hidden gems? :)