Few day side trip from Lake Constance with a teenager in December

You can take the train, just a little over an hour, to Lindau which has "one of the best" German Christmas Markets until the 22nd of Dec 2021. It has over 100,000 lights and 150 stalls and a 360 Ice Bar on a moored Ship. You can buy a 3 Day Pass, a Bodensee Card, at the Tourist Information in Lindau that covers boat rides (some run only from April thru October) (some sail only on Advent weekends) and admissions to 160 sites in the 4 connected countries. I found this information online, it looks like something that could work for you.

From Überlingen, there is very good rail accessibility all along the north shore of Bodensee, to the towns of Friedrichshafen (26 min), Lindau (36min-1H), and Bregenz, Austria (10 min from Lindau). There are also many cruise boats running on the lake.

Friedrichshafen has the Zeppelin Museum. Very interesting, particularly for someone technically inclined.

Ah, Lindau. One of my favorite places in Germany. A lovely harbor. Look out across the lake to mountains in Switzerland.

Just beyond Lindau is Bregenz, Austria. There, the Pfänderbahn is a cable car the goes to the top of a mountain east the city. From there on a clear day you can see down the lake almost to Konstanz.

There is a boat connection from Überlingen to the island of Mainau.

Sigmaringen, with its Hohenzollern Castle is about 2 hours by rail for Überlingen by way of Friedrichshafen.

The Mainau is great for "beautiful views that don't require too much effort". December might not be the greatest time for the flowers, but they also have a butterfly house, and maybe they have special events for the Christmas season.

Then there is Meersburg for your castle and a nice old town on the lake.

More beautiful views with not too much effort from the Pfänder which has already been recommended, or from the Säntis at 2500m.

For something eccentric and cultural at the same time, visit the prehistoric pile dwellings, UNESCO world heritage.

The Rhine Falls at Schaffhausen cannot compare to Niagara but they are impressive.

Stein-am-Rhein, reputed to be the prettiest town in Switzerland.

There's plenty to do round the lake for all your time there, and I'll throw St Gallen into the hat.

If I read your post correctly, you are looking to get away from the lake for a few days. All the places I mention should be decked out nicely for Christmas, though some of the smaller places only have markets on weekends (some not at all).

A: Garmich-Partenkirchen, about 4 to 5 hours away by train from Überlingen. From here you can reach a number of good spots by public transport, within an hour or so - Munich, Füssen (and the famous palaces close by), Oberammergau, Ettal Abbey, Linderhof Palace, Andech Monastery, Mittenwald, Innsbruck (Schloss Ambras). GP has a couple of nice gorges, a nice, easy path leading to and from Grainau. I actually prefer Füssen and Mittenwald, but GP is more central and cuts down on daily travel time. I also don't like changing bases too often.

B: Colmar, Alsace, (presuming border crossing is easy), about 4 hours away from Überlingen. Buses from here to all the worthwhile villages close by - Kaysersburg, Eguisheim (different buses); Riquewihr, Ribeauville and Bergheim (on the same bus route). Note, Bergheim is a non-touristy, very attractive village. You can also take a train to Freiburg, a university town which has a nice old core. Strasbourg is about an hour away from Colmar, as is Basel.

There are other places you can spend five days, but I'd choose of the two above. PM me if you want further information. Try this website: https://www.bahn.de