The Marais is one of the oldest & most interesting areas of Paris. Located in the 4th, it was traditionally the Jewish area of town, and still houses numerous temples for the community. On Friday evenings you see many men, dressed in the traditional Orthodox clothing, walking to temples for services.
This area also houses the Hotel de Ville of Paris (the city hall), the Paris city museum, the Pompidou Center, the Place des Vosges and probably 50 other things to see. More recently it has become quite the area for night life, and one of the few areas good to shop in on Sundays (most of the stores are closed on Sundays in the other neighborhoods).
We spent a sunny Saturday afternoon fighting thru traffic (I would recommend NEVER driving & trying to park there - too many small one way streets & crazy Parisian walkers), but having a great lunch and some fun around the fountains. We then had to go back the following week (on the metro) for a more leisurly stroll down the streets.