I like very much this part of the city: jardin des plantes where you can also find l'Arene de Lutece. Near there is also Rue Mouffetard, another place very old, by Roman age and there you can fin one of the best creperie in Paris!!
A very nice street with lot of restaurants, wine and cheese stores, small shops... There are people in the street until late in the night. This place just feels good.