16 nights in France in December

18-21st Paris
22-24th Colmar
25-29th Nice
30-2nd Paris

I would take nights away from Paris at the beginning and add them to Colmar. Strasbourg is well worth a day trip, especially if you haven't been there before. The Alsatian villages around Colmar are especially charming then - note that some may close their markets a few days before Xmas. 2 days in Colmar simply isn't enough. I'd consider going straight to Strasbourg and putting all your Paris at the end - the markets in the other places will be better and the lights and decorations will probably be around till after you leave. The problem is where to spend 24-25th. Do some research to see what's open. You don't want to arrive in Nice late in the day and find all the restaurants closed and nowhere to eat (or buy food) for the 25th. I don't know how you're planning to get to Nice from Alsace. Rome2rio.com shows 8+ hours by train. The fastest is to fly from Strasbourg.

I'm planning to visit Nice in mid-Dec for the markets and decorations on the Riviera. My research says they are well worth seeing.

I agree that an extra day in Colmar would be good!
But you do have to stay in Paris at the beginning because you get there in the evening. Could you fly out of Nice ? This way, you could spend all your Paris time at the start.

You can make the most of your time and save money if you stay at the departure city for flying home at the end of the trip.
For this itinerary, that means staying in Paris at the end of the trip rather than both at the beginning and again at the end of the trip. And that means that you'll still be in Paris for New Year's Eve and Day.

Here's how I would do the itinerary-- Take a high-speed TGV train directly from CDG Airport to Strasbourg when you first arrive. The train takes less than 2 hours. You could overnight in Strasbourg, a great university town and home to the EU's Parliament, and take the 30-minute train the next day to Colmar. Eguisheim is one of those smaller towns of old world charm you can easily visit from Colmar on the local bus that takes all of 10 minutes to get there. ( There's so much of interest in the Alsace-Lorraine that a strong case could be made to extend your time there a night or two.) After Colmar , there are flights from Strasbourg's small Airport (SXB} to Nice on December 21 for $43 per passenger; December 22 for $63; December 23 for $74. ( www.Skyscanner.com).
Returning to Paris from Nice can be by TGV train in 5.5 hours or by air.
There are TGV trains from Paris to Carcassonne that also take about 5.5 hours. But you can visit the chateau of Chantilly and then the preserved medieval town of Senlis from Paris all on the same day and be back in Paris for dinner. The train goes from Paris Gare du Nord to Chantilly in as little as 25 minutes. From Chantilly, bus 645 takes just 20 minutes to reach Senlis.
For the best deals on the TGV train tickets buy them online as soon as you can at www.sncf-connect.com You may have to wait until December 11 after SNCF releases the new train timetables on December 10.
Have a great time in France!

Thanks for all your wonderful feedback.

The idea behind having a split stay in Paris was to be there for Christmas and New Year, but I think your suggestion that I consolidate and perhaps reduce our stay in Paris, upon reflection, makes sense. We have never been to the Alsace region, and it is clearly a terrific place for Christmas vibe/markets. Modifying the itinerary accordingly...

18-22nd, 5 nights (4, really) in Alsace
23-27th, 5 nights in Nice
28-2nd, 6 nights in Paris

My preference in this case would indeed be to head out straight for Alsace (although, we might not get to Strasbourg until 10-10:30pm, later for Colmar.) What if we spent 2 nights in Strasbourg, giving us one full day there, and 3 nights in Colmar? (I gather Colmar is the place to use as a base in Alsace.)

Please let me know what you think.

@Chani @Kenko
I had not looked at the train travel time from Alsace to Nice so thanks for pointing out what's involved here. I will certainly consider the option to fly. One of the charms to me of being in Europe is public transportation/train travel. Train journeys allow one to unwind, enjoy the view/changing character of a country without exhausting oneself, while also enjoying a good book maybe. Still, 8+ hrs. is a long time.

@balso
Our tickets are already confirmed, so it will be difficult to change it to open jaw now.

@Kenko
Love, "Citizen of the World"
Thanks for your recommendation of a day trip to Chantilly and Senlis. That's a wonderful suggestion!

Excellent to stay a couple nights in Strasbourg before going to Colmar. In 1988 UNESCO named the entire historic city center— an island called Grand Ile — a World Heritage Site. Strasbourg’s Cathedral, Quai de la Petite France, art museums and dining are highlights in the Alsace-Lorraine’s main city of 250K. And your timing means you’ll be able to enjoy Strasbourg’s famed Christmas Market since it will be running throughout December up through 12/23.
Have a great Trip!