After flying in to CDG I would probably stick with the easiest to get to by direct train (with no changes). Of the suggestions above, Reims would be my choice. It's still a large city but you'd likely only be visiting the old center and, as stated above, the cathedral is historic and well worth visiting, along with the champagne houses if that's of interest. Also,since you'll be there in December, there will be a fun Christmas market in the plaza in front of the cathedral. My second choice would be Nancy, but it requires a longer train ride and a change of trains in Meuse. I also think Chartres would be wonderful but it requires either train and metro changes in Paris, or bus from CDG to train station in Paris - unless there's a direct bus or train that I'm not aware of.
All of my comments are dependent on whether or not you want to rent a car of use public transportation. With a car my first choice would be Chartres, then Nancy, Strasbourg, or Colmar - Chartres or Colmar for the smaller city feel. With a car I might also consider Amboise, Chinon, or other Loire valley towns and visit some chateaux.