Is there anywhere with lockers - or a place to leave bags - during a day trip to Paris?
Are you arriving in Paris by train? if so, train stations usually have baggage lockers.
Gare du Nord, Gare de l’Est, Gare Montparnasse, Gare d’Austerlitz, and Gare de Lyon have luggage storage. There are a number of businesses that do as well: Holibag, Nannybag, Eelway, and City Locker.
Thank you both - issue solved!