Suggestions for one day left in Paris

Well, maybe Chateau Malmaison which was Empress Josephine's favourite abode. You can get there by bus from La Defense in about 15 Minutes. It is seldom crowded and makes a nice half-day expedition.

Have you been to the Nissim Camando museum? The Jacquemart-Andre? Either one is well worth the couple of hours the visit would take.

Have you been to St. Denis which is reached on the metro? And there are tons of interesting towns near Paris if a day trip feels like fun (note that Mondays tend to be shut down days for towns ). You can see snapshots of St. Denis and a few of these nearby towns here: https://janettravels.wordpress.com/category/day-trips-from-paris/

And if this feels too ambitious you might get ahold of a good Paris walks type book and explore a neighborhood you haven't seen. We enjoyed a walk in Butte Aux Cailles and there was a really good architecture walk in the 16th in one of the Frommer books with Paris walks. I also once did a walk of the major graffiti sites in the 19th and 20th that was really fun. My snapshots are in that same photo journal under 'Paris' and the URL for the map is there, although I did this about 6 years ago and graffiti sites do change rapidly.

Have you been to niche museums like Nissim Camondo, the Marmottan (which has the Monet for which the impressionist movement was named, 'Impression Sunrise,' the Cluny, the Jacquemart Andre etc etc

I'd be inclined to walk around with the structure of some walks books and get to know Paris a bit better in a leisurely way.

I am also a fan of the museums Norma mentions , and Janet's advice about St Denis is spot on . If you have limited time during the day , here is another to consider _ Les Gobelins - https://en.wikipedia.org/wiki/Gobelins_Manufactory and http://www.unjourdeplusaparis.com/en/paris-culture/visiter-manufacture-des-gobelins