what are best things to do in Paris on Mondays?

On our recent trip to France, on Monday, aside from the usual closings, many other locations were closed for the Mayday celebration. We went to Chartres (by car for us, but public transportation there is quick and easy), visited the cathedral and had lunch. Then we returned to Paris, where we went to the Luxembourg Gardens for a relaxing afternoon watching Parisians enjoying their lovely city, and followed up with a stroll through, and dinner in, the Latin Quarter. We took a cruise on the Vedettes de Pont Neuf that began just before sunset, about 9:00 p.m. I ordered our tickets ahead of time on their website to reserve a particular time. They were cheaper than buying them at the departure point. Just print the reservation and show it at boarding time. All in all, a lovely day and evening.

I arrived in Paris on a Monday and visited the Pompidou, Notre Dame and Conciergerie plus walking along the Seine and exploring nearby areas.

Paris Walks offers great neighborhood tours for 15 Euros a person.

Versailles is also open on Mondays.

No, Versailles is closed Mondays, except for the gardens. Many museums and churches are open, including the Louvre and Sainte-Chapelle.

There are quite a few sights available in Paris on Mondays. They seem to have realized it's better to have some places close on Tuesdays in stead to encourage visitors every day of the week.

Thanks for the advice. I've dove into the RS Paris book and have come up with some. Anyone know how busy the Louve is on Monday? If you are there when it opens, can you escape the masses? I think a nice stroll thru the Pere Lachaise cementary in the afternoon and a tour on bus #69 looks like a good Monday.
I would have gone to Montmarte, but booked a secret food tour for tuesday. Going to Versailles on Wed.
Does anyone know if you can still go in Notre Dame on Sunday morning or do the close it for mass?
And yes we will be purchasing a museum pass.