It might be too late for this message to reach you (since you're en route to Paris or already there), but if you're looking for a neighbourhood to explore for drinks and dinner where you are more likely to meet people, I would recommend that you head to Oberkampf. (We liked Chez Justine.) It has a younger, primarily French crowd and is a bit of mixed bar/restaurant scene - perhaps comparable to the Lower East Side in Manhattan.

How early to do you have to leave for the airport on Sunday? If you have a few hours in the mid-morning, the Marais for brunch is a good choice, but if you have to be at the airport by early afternoon, you won't have enough time.