Just walk the main avenues. From Arc de Triomphe via Champs Elysées towards the Louvre. Then take Av. de l’Opéra and walk past the Opéra. Galeries Lafayette should probably be decorated with Christmas lights in not so long.
Usual suspects. See a show at the moulin rouge. See the architectural sites like the Arc de triomphe, Eiffel Tower (skip the ride up the top. Not worth it. Impressive enough from the ground), meander around the side streets of the 8th, 7th, 1st arr. My favorite is just enjoying coffee or a beer and some pastries on street cafes. The Sacre’ Corur is amazing. Recommend walking to the top.
Get used to the underground metro on day one. It’s fantastic. Versailles is really nice if the weather is good. And as with the louvre, buy tickets ahead of time and reserve your time slot.