Rue De Rivoli will put you pretty much in the heart of things. You probably won’t see much activity at Christmas, since France observes the holiday like much of the western world. Paris can be pretty cold, wet and windy in winter, but perhaps you’ll get lucky.
The city sprays the trees white along the Champs Elysee and covers them with tiny white lights, which is pretty.
I’m no expert, but I would go to a good bistro and stoke up on good sturdy warming cassoulet. I read that the restaurant Le Thoumieux, in the 7th arrondissement, makes the best in Paris. That’ll keep ya fueled up and warmed against the French winter.