Visit one of the Lenotre pastry shops. If you go to the one by the opera house, there are a number of speciality gourmet shops in that area.
Musee d’Orsay is my favorite of the museums. If you can, eat lunch in their restaurant (not the cafe, the actual restaurant). Feels funny to recommend a museum restaurant but the food is outstanding and it’s an amazing space.
I love just walking around Paris. Love the area near Parc Monceau. I could stroll the city streets all day for your 12 days and not get tired.