While there is still some sightseeing you can do when the sun goes down, it's probably better to put that aside and enjoy Paris' nightlife.
Almost every neighbourhood has something special to offer. However, you probably don't have a lot of time in Paris, so I'll list 2 places that should be high on your list:
Both of these are in the
This is a popular hangout with "right-bank" types and it's an excellent place to unwind during the week.