Suggestions for one day in Paris

Where does your trip end and at what time?

If your flight leaves at 6 pm, I think it is suggested that you get to the airport 3 or even 4 hours early.

, I’ll have a big backpack to schlep. Know where I can store it until I leave?

Are you sleeping in Paris the night before? Most hotels allow guests to store their luggage for the day when checking out. If not staying at a hotel, see if the nearest train station has luggage storage. Note that you really dont have most of a day. You need to be at the airport 3 hours before a long haul flight, so you would need to leave by 2:00 or 2:30 depending on how you plan on getting to the airport.

Its not easy to recommend activities without knowing your interests. If there are any of the major attractions you haven't seen, then do that in the morning. Perhaps a long last stroll along the Seine and check out the bouquinistes, followed by a leisurely lunch at an outdoor café, watching the world go by.

There may be some major traffic issues on the 25th. The Pope is scheduled to visit Paris Sept 25 & 26. Depending on what you want to do, I’d figure out your transportation out to Orly and then stay in the area possibly near a Metro Line 14 station that will be open that day. So far the Papal activities I’ve seen are at Notre Dame and Place de la Concorde on the Right Bank so I’d avoid that corridor.

There were reports Friday that the papal mass will be on Friday the 25th on the Champs-Elysées -- my guess is, probably extending down to the Concorde.
EDIT: The source below says the mass will be on Saturday the 26th. Friday will be arrival (at Orly!), Notre Dame, Elysee, UNESCO, Stade de France ....

Anyway central Paris will be a mess. You are very smart that you gave your date in your post as it changes everything !!

So yes you will want to be well clear of central Paris that day.

Here's an article (in English) from the official Paris tourism office about what is known so far about Pope Leo's visit to Paris. Of course, many details are still being finalized.

Pope Leo XIV's visit to Paris in September 2026 https://share.google/xBKWpz3wGb2WU8T2u

Luckily, while he arrives at Orly the same morning you need to leave from there that evening, he doesn't depart from Orly until Saturday.

For storing your bag, if you don't have a Paris hotel where you are staying the night prior, I would find a NannyBag or Stasher sight somewhere on the Left Bank near a line 14 metro stop.