Dancing Sculptures along the Seine
This is my favorite palce for pic nics. The Quai Saint Bernard has dancing lessons on Salsa; Tango, Rock n Roll for free all summer, and the park/open air sculpture area are just beautiful to look at and enjoy. The Institut du Monde Arab is just nearby, and you can go to the top of the building (for free) and get a peek of Paris from a decent height!
Quai Saint Bernard, métro Jussieu, then walk towards the Seine (ask!)