Between Christmas and New Year's Itinerary Advice

Koblenz is a good base-town choice for you. Trier, Mainz, and Cologne are all feasible destinations for day trips. There are direct train options from FRA to Koblenz.

The Rheinsteig trail and the Rhine Castle trail are good options in this area. Trains will get you to different towns for hiking different stages of these trails.

Braubach's Marksburg Castle (sorry, not ruined, completely intact since the Middle Ages) is open year-round for tours. From Koblenz you're around 10 minutes from Braubach by local train.

Map of the Upper Middle Rhine Valley with rail and ferry crossing details:
http://www.loreley-info.com/eng/rhein-rhine/walking-hiking.php

Yes, most Christmas markets will be closed. They usually close around the 22nd or 23rd. The point of a Christmas market is to be a place where you buy things you need for the holidays, food, decorations, gifts etc. So there is really no point in keeping them open after Christmas. Some might do for tourists, but that is not very authentic.

You've chosen a time when smaller cities might not be deserted, but certainly a lot calmer than usual. And you might want to check the opening times of any museums you're interested in.

I would concentrate on where you want to be for Silvester or New Years. Dinner, party, fireworks etc. I think you will be surprised. Many people buy a lot of fireworks and shoot them off here. It ca be really something else.

Salzburg markets are open thru Jan 1. Perhaps visit Munich and spend a few days in one of my favorite cities?
Safe travels!

We will be in the same area at the same time. This is what I found (below) regarding some holiday-specific things. Please double-check everything I am writing here, because a lot of this was based on articles I found describing the celebrations in prior years, and I'm assuming it will be similar next Christmas also.

Strasbourg - the nativity scene in the cathedral is supposed to be really neat, and there through at least Epiphany (Jan 6)

Colmar - supposedly one of the best christmas market cities, and open through Dec 29. (I did confirm this via email with the tourist office in Colmar).

Salzburg - a more organized New Years Eve celebration compared to most anywhere in Germany. Formal city-sponsored fireworks over the old town area (or maybe the castle/fortress area, I'm not sure, but they're in the tourist center), along with a stage that has music/concert and festivities and a screen that displays other cities' NYE celebrations each hour (other cities which are located to the east and celebrate before Salzburg). Some Christmas markets still open through New Years.

Outside Salzburg, the St Gilgen area (in the Alps) has some kind of tram that goes up the mountain, with many statues/figurines showing the journey of the Three Wise Men along the way. (I saw this mentioned in a 2022 publication; it was new in 2022 and I didn't totally understand what it was describing (partially because I was reading a poor translation of something in German), but it sounded like many statues that depict the three wise men's journey, located along a scenic tram ride and hiking trail)

Thank you! Your suggestions are great. Thank you for doing the legwork of confirming with the travel office in Colmar. Christmas markets have been difficult for us to pin down in the past... Brugge said they were done Dec 24, but were still doing their thing until Dec. 26. The opposite was true in Salzburg and Vienna. It's all part of the fun. :)

We've been in Salzburg around Christmas twice and it was a delight. I have no doubt their NYE celebration will be excellent.