From may to October, you should go to the 'quai de seine', between the Seine and Jussieu/jardin des plantes. It's young, but you'll see families, a lot of people learning to danse and picnic. With the occasional rose/wine seller. Probably the Parisian place I hate the less, with 'Chez Amel' near Canal st Martin (La station is fine also but too snob for me).