I second St Sulpice (although when I was there a few months ago, the left half was covered for construction), as well as the Sacre Coeur, which has magnificent architecture, and a really charming and quaint surrounding. (Cafes, shops and artistes in the pedestrian-only sidewalks!)
I went to Galleries Lafayette in Mont-Parnasse and Nice and was sort of disappointed, but I suppose it depends on what you're looking for. I picked up some tea leaves and mustard at Fauchon - skip the madeleine biscuits though - they popularized them, but I found them greatly disappointing. Dry, tasteless, with the appearance of a cupcake that wanted to rise, but spread sideways instead.
I ate at the famous La Rotonde in Mont-Parnasse. The food was passable, but it's really only worth eating there for the history, ambiance and good service. I also ate at Les Deux Magots at St. Germain-des-Pres for my last night in town, and that was fantastic.
I think the St. Germain area is promising for food, no matter where you go. And Mont-Parnasse has blocks of ethnically rich and diverse dining opportunities too - Japanese, next to Chinese, next to Korean, next to Thai, next to a full avenue seemingly dedicated only to 'creperies'!
Can't offer much in the way of museums. I only had time for the Louvre

. My only advice is to allocate at least a full day to just the Louvre, and do your own Da Vinci tour, as Kissy suggested. (I was trembling with anticipation in St. Sulpice, and there's even a sign by the obelisk that dispels the myths from the book!)
"It's not always about you!!!" (But if you think I'm referring to you anyway, then I probably am.)
"Good luck returning my ass!" - Wilhemina Slater
"This is my breakfast, lunch and f***ing dinner right here. I'm not even f***in' joking." - Colin Farrell