Every summer for the past dozen or so years, Paris has turned areas along the Seine, along the Bassin de la Villette and in front of the city hall into sandy beaches for the summer, under the name Paris Plages. Above, along the river near the Hotel de Ville, below along the Bassin.
If you can’t take the time to get into swimming gear and lie in the sand, there’s still relief. On hot days, it’s amusing to see fully-dressed (including suits) people walking through these mist stations to get a little relief.
And for the more active crowd, more athletic activities, just next to the Bassin beach.