If you have to do shopping for Christmas and would like to do it without stress or being pushed, I would suggest avoiding the weekend, especially in the main stores.
You can try the Parisian Christmas markets, a new "fashion" occurring in Paris for the last few years, but I don´t think you will enjoy them all that much, because they'll have the same small houses as in Alsace, for example, and they don´t have handmade production and exist only for a more commercial aim. Sorry, but the "Christmas market" of the Champs Elysées isn't worth the trip, in my opinion.
Are you interested in Christmas music ? Here are some concerts where I'll certainly be in the next weeks
December 20th (free) and 30th , Christmas concert at Notre Dame de Paris
Others announced on this website http://www.spectable.com/agenda-culturel/concert/musique-classique
and many other free concerts in the parisian churches here : http://www.paris-fete-noel.fr/Pascale