I believe we saw the Marie Antoinette exhibit at the Grand Palais on a Monday. Seems all the museums were open on Sunday - even Easter Sunday. I believe the Louvre is closed on Monday or maybe Tuesday? But the Louvre and D'Orsay close on different days so you can plan around that.

gruezi

My favourite things to do on a Sunday in Paris:

Morning: wander around the market on boulevard Richard Lenoir (near Bastille) and then have an aperitif at a pavement café somewhere in the vicinity.

Lunch at Bofinger (or l'As du Falafel if feeling cheap).

Afternoon: wander round the Place des Vosges and the Marais where plenty of shops are open.