Reaching out for off the beaten path/under the radar suggestions for an upcoming month long stay in Paris. Having been four times over many years I've been to the major museums and attractions and am interested in others recommendations for small museums, discovering new neighborhoods, day trips and the like. I've got overnights planned to Lyon, Dijon & Bayeux. Day trips to Strasbourg & possibly Auvers-sur-Oise.
Thanks in advance, I know there will be great suggestions for the folks on this forum

Any reason for spending the entire month in Paris? Strasbourg is a wonderful city with some beautiful day trip options. I would make it a longer stay there.

Have you been to the south? Nice is another wonderful city with some amazing day trip options.

Chose to stay in Paris for the month for a number of reasons primarily loving the city and wanting to leisurely enjoy it but also the flexibility of easy train travel. Strasbourg will be a day trip as well as a couple of other overnights.

A few years ago I ran across this site:
https://www.unjourdeplusaparis.com/en/

There are tickets and tours for sale there, but if you look past those offers you will see many suggestions for unusual sights, street and neighborhood walks and museums. I think it was on this site that I found the Notre Dame du Travail church which we so much admire. This link takes you to the English language pages. Unfortunately some of the translation from the French is clumsy so if you can read French, just change the language at the top of the page. I'm envious of your full month stay. We have visited Paris many times and have never run out of new and interesting things to do.

Thank you so much MaineGG. I will definitely check it out.

Some advices are subject to your dates which you didn't mention. For example Giverny is a must but close for the winter. Mont St Michel is not close but I would'nt travel there on the winter when the days are short (some would even say it's crazy for a day trip but I did it and enjoyed it a lot).
Strasbourg is wonderful but if you have the time I would spend a day also in Colmar.
Strasbourg and Mont St Michel are something loke 2.5-3 hours long from Paris so you might considering longer satying around though a day trip is certaily doable (writing from experience).
The Loire Castles will require you to stay in the region (Tour for example) but you can easily visit Blois as a day trip.
There is a lot to see in Rouen and Reims.
Fontainbleau is exquisite, so is Chantilly. Chateau de Maintenon is very nice and a bit off the track. The cathedral in Chartres is very famous but an unforgettable and quite unexpectable attraction there is Maison Picassiette. Utterly breathtaking.
As for museums - Hotel de la marine (quite new), Bourse de la commerce (also quite new), Cluny (recently renovated), Hotel Carnavalet (recently renovated), 2 ethnic museums are Quai Branly and Guimet, Fondation louis Vuitton, Musee de la vie Romantique, Hugo's House, Cognyac Jay (free!), Jacquemart Andree (recently renovated), Maillol, Rodin, Musée de l'Orangerie, Musée des Arts Décoratifs, Musée des Arts et Métiers, Musée d'Art moderne de Paris, Maison Europeenne de la Photographie, Atelier des Lumieres, Cernuchi, Musee Gustave Maureau, Musée Nissim de Camondo - and these are of course add to the big and famous museums.

Thank you for the many suggestions Zvi! I will be there starting mid-March, optimistic it will be more mild. Thankfully I've visited MSM in spring but am heading to Bayeux for a couple of days and preparing for some weather. Definitely some museums on your list to visit. Thanks again.

My favorite place in Paris is Les Puces de St Ouen. A collection of about a dozen flea markets, many of them antiques/vintage , which can range from a suit of armor to mod store mannequins. It is a riot of obejts d'art. Marche Vernaisson is my favorite. Best visited on a Saturday or Sunday morning. But you did not say what time of year....

Marche des Enfants Rouges is an open-air market with some restaurants sandwiched in.

Stores:
Marriage Freres on r. Bourg-Tibourg for tea, a visual delight. The MArco Polo tea is awesome.
Jacques Genin for chocolates and, if you are lucky, maybe a pastry or two. Personally, I would order a tarte citron-basil in advance - feeds four, but you are there long enough! The inside looks more like a jewelry store.

Climb the stairs of Sacre Coeur to walk along the rooftop.

Thanks, shelemm. Flea market is a great idea. Read about it but never made the trip to it.