One afternoon in Paris: what would you do?

I’m going to Paris on Sunday. I have meetings for three days, but I’ll have Sunday afternoon to myself. I arrive at Gare du Nord just before noon. I’ve been to Paris a few times, so I don’t need to do anything terribly touristy.

If you had a single, chilly afternoon (probably around 10 deg C) in Paris, what would you do? Musee d’Orsay? Roman ruins in the 10th? A museum I don’t know about? Or what am I missing?

I’m definitely a fan of history museums, but I’m open to any suggestions. This is my third time in Paris this year and I’m absolutely set on doing something worthwhile. Thanks in advance.