Last summer when we were in Paris, we checked out the recently opened les Berges. It is an approximately 1.5 mile promenade along the left bank of the Seine stretching from Port de Solferino to Port de Gros Caillou. It is a nice place to enjoy the Seine against a backdrop of greenery that has a more contemporary feel than many of the traditional gardens in Paris.
You can also grab a casual meal or drink from one of the several outside dining options (including a food truck or two) along the walkway.
In the mood for something a little more active? There are a number of recreational opportunities from checkers to yoga. My daughter loved the swing ball game (sort of like tetherball) and scaling the small climbing wall.
P.S. When we were in Paris last winter, les Berges was pretty quiet. But I just received a newsletter from them and it appears that things are heating up again. So be sure to check online at les Berges for specific dates, locations and activities.
