Seine
Summary: Part of the Seine was converted to a beach for one month in the summer starting in 2002, with permanent conversions coming later in 2013 and 2016 to turn the former expressway into a pedestrian/cycling path and public park. It was initiated by Bertrand Delanoë and later followed up on by Anne Hidalgo, with the intention of reducing pollution in the center of the city and creating more public space.
Date: Temporary Beach 2002, Left Bank 2013, Right Bank 2016
Sources:
https://www.dw.com/en/paris-oks-pedestrian-only-zone-alon
https://www.dw.com/en/the-seine-river-banks-in-paris-now-car-free/a-38300589