Side trip to Germany from Paris...which town(s) should we go to?

I am going to suggest Mainz - it's not all that far from Paris and you have the advantage that you can base there and see all the other cities in the Rhein-Main district, as well as taking yourself up the Rhine. Rüdesheim, Wiesbaden, Frankfurt, Boppard, Koblenz, Cologne, Oberwesel will all have markets. There might be some smaller ones that I have missed. If it's still too far for you, there is Stuttgart, but it's not my favourite city. However, you could base there and see smaller markets like Esslingen.

Lavandula

Thank you!!

We lived in Augsburg and enjoyed its Christmas Market, but never visited other cities markets.

Aside from Christmas Markets, I would recommend Strasbourg, France (on the border with Germany) and nearby is the Black Forest (not sure. that is good in Winter).

You might like Heidelberg. Nice smaller city on the Neckar River, daily Christmas market, easy to reach from Paris...

Direct TGV trains from Paris into Germany reach Mannheim in 3 hours or less. From there it's about 15 minutes by connecting train to H'berg. Random date of 12/4 shows a current saver fare price of €44 (at the DB site) from Paris to H'berg at 9:06 if purchased now.

Day trip to Bad Wimpfen by train for Christmas market? Dates at site below:
https://weihnachtsmarkt-badwimpfen.de/

Speyer (direct train from H'berg) is an important historic town, less than an hour from H'berg by direct train. Daily Christmas market runs through Jan 6.

Just in case you want to use Public Transport->Trains

TGV to Karlsruhe via Strasbourg(direction Frankfurt) 2,5 hrs

From Karlsruhe 15 min to Baden Baden (direction Basel) to see the Black Forest and use Spa

For free Public Transport in the Schwarzwald Region (Black Forest)
check accomodation that offer Konus card

or

Still in France via Strasbourg visit Christmas Markets in Colmar (30min) and/or Riquewihr etc

Cologne is closer to Paris, has one of the largest Christmas markets in Germany plus some smaller markets (link) and is a good base for day trips, e. g. Aachen, Brühl / Augustusburg, Koblenz, or Bonn and Königswinter incl. Drachenburg Castle. Cologne museums vary from Roman collection over arts to chocolate.

Thanks everyone! This is SO helpful!

If you make it down to Colmar and Strasbourg for their Christmas markets, I would suggest heading over the border to Gengenbach, which is about 40 minutes by train from Strasbourg, and visiting this adorable and charming town. Gengenbach not only has a wonderful Christmas market, but it also has the world's largest Advent calendar displayed on its town hall. And the town itself is just gorgeous, with half-timbered houses and lots of wonderful cafes and restaurants.

Here's a quote from another thread about German Christmas markets (and the Gengenbach market in particular) that you might find useful:

I really liked Gengenbach, Germany. Small town and small [Christmas] market. But the town is ridiculously cute. We were there around 4:30 on a weekday and it wasn’t crowded. The stalls are along a wide street, and all around the central square. So they are not crowded together as in some places with tons of stalls. There were unique items for sale - both food and crafts. The main difference at this market was the space and lack of crowds, in comparison to places like Strasbourg which was packed shoulder-to-shoulder in the evenings. But the buildings were just as charming. Quoted from: https://community.ricksteves.com/travel-forum/general-europe/best-small-town-christmas-market

While my list leans towards larger cities, it's not 100% clear that you want only small towns. I happen to like museums, so this list will not help you pretend you are in medieval Germany. I personally might use a car for multiple small towns in a day, although I don't enjoy driving in foreign countries. This list is deliberately northwestern Germany oriented. You'll want places to warm up at this time of year.

Aachen
Cologne
Dusseldorf
Wuppertal
Essen
Munster

If you intend on deviating for 5 whole days to Germany from Paris, I suggest also choosing from some of those towns listed: Aachen, Münster, Neuss am Rhein, Cologne, maybe even Soest/Westf. depending on how far you want to go.

The train from Paris to Frankfurt takes just a bit under 4 hours. If you hub here, you have access to markets in medieval towns like Limburg, Marburg, Seligenstadt, Michelstadt, or cities like Mainz, Heidelberg, or Wiesbaden. Even Rüdesheim on the Rhein is just a bit over an hour by train. If you are here on the right weekend, the Christmas markets in Büdingen or Idstein are great. The one in Bad Homburg which is only on weekends, has the most hand-made items at a market. All of these towns are with-in hour or so from Frankfurt by train.

The Frankfurt market has 230 stands and has its' beginnings in 1393. It has spread out over the years and is a lot nicer than it used to be. If you are here on Wed. or Thur. night, a brass band plays from the gallery of the Alte Nikolai. Most of the downtown churches have concerts of some sort almost every evening - organ, hand bells, choirs, brass bands, etc.

@Tim, you suggested we will need to warm up. Is northwestern Germany warmer than southwestern?

Is northwestern Germany warmer than southwestern?

not usually. Baden-Württemberg is the sunniest spot in Germany, statistically

but all days and years are different - no guarantees

As someone who lives 2.5 hours east of Paris by train these are the cities I recommend for the Christmas Markets on the German side of the border.

Trier - good market, great cathedral, fantastic Roman ruins and historical museum. Trier was the largest city in the Roman empire outside of Rome itself until 400ad. 3-4 hours by train. 4 by car.

Koblenz - good market, the Eck, the winter garden at Fortress Ehrenbreitstein. 5+ hours by train.

Mainz - Good market, lot's of history. 4.5-5 hours by train.

Heidelberg - Better market(s), great castle. 3.5-4 hours by train

Those are the bigger cities. There are hundreds of smaller markets around, but some of them only run one weekend. You could easily go to Trier and then up the Mosel to Koblenz and hit a market a day (Bernkastle-Kues, Traben-Trarbach, Cochem, etc.)

Munich, Nuremberg, Berlin, etc. are plane rides, not train trips, from Paris.

https://weatherspark.com/m/56241/12/Average-Weather-in-December-in-Freiburg-Baden-W%C3%BCrttemberg-Germany

https://weatherspark.com/m/54495/12/Average-Weather-in-December-in-K%C3%B6ln-North-Rhine-Westphalia-Germany