It's a must whether you view it from Champ-de-Mars or Trocadero or even go up to take in the view of Paris.
If it is a cold or rainy winter Paris day, check out the Louvre, Musee d'Orsay, or Centre Pompidou.
Take a walk down Champs Elysees & Boulevard Hausmann and stop at Galeries Lafayette. Visit the Christmas market & ice skate under the Eiffel Tower too.
Take a day trip or half day trip to Versailles. Even though the gardens aren't in bloom, it is still beautiful.
Bring lots of layers. Don't expect snow. Bring a rain jacket. Be flexible so you can adjust for the weather.