We’re definitely interested in WW2 historical sites. That’s why we
were going to Berlin but may decide on somewhere else based on your
suggestions.
The Normandy Beaches are very worthwhile IMHO. By train, go to Bayeux for a couple of nights from Paris. Look into tour options from there or rent a car.
Trains connect Paris to Cologne and the Rhine/Mosel. WW II options there:
Cologne: NS Documentation Center (There are doc-centers in Munich, Nuremberg, and Berlin too. This one is housed in the former Gestapo Headquarters building.)
South of Cologne in Remagen: The Bridge at Remagen was partially filmed at this site, where nowadays you can visit a museum housed in the supports that remained after the wartime destruction of the bridge.
http://www.bruecke-remagen.de/index_en.htm
Definitely interested in concentration camps, museums, churches,
cathedrals, castles, etc. would love a cable car to somewhere, just
need to narrow it down.
Cologne/Rhineland area has some of Germany's top cathedrals/churches:
Cologne Cathedral
Aachen (near Cologne) Cathedral
Mainz (On Rhine near Frankfurt) Cathedral
Trier (Mosel River) Cathedral
The above cities have a good selection of museums as well.
Castles:
In the Middle Rhine Valley (south of Cologne and Remagen) Marksburg and Rheinfels (ruins w/museum) are your best bets for a tour. The others on this map tend to be hotels, ruins, private property. Click on castle names for more information:
http://www.loreley-info.com/eng/rhein-rhine/castles.php
In the Mosel Valley (which intersects the Rhine at Koblenz) look into Burg Eltz (near Moselkern) and Reichsburg Castle (in Cochem) for tours.
Burg Eltz:
https://classroom.ricksteves.com/videos/germany-s-mosel-river-eltz-castle-and-feudalism
https://burg-eltz.de/en/
Reichsburg:
https://www.german-way.com/travel-and-tourism/germany-for-tourists/castles-and-palaces/cochem-castle/
https://reichsburg-cochem.de/?lang=en
https://www.youtube.com/watch?v=lGiaFS099bg
Cable car/lift rides are popular in many scenic spots. In the Rhine and Mosel Valleys...
Cochem:
https://www.moselcamper.de/wp-content/uploads/2018/03/sesselbahn_5.jpg
Boppard
https://img.marcopolo.de/api/content/images/image/0/3/tf/ei/t1024/u/photo-6411834.jpg
Rüdesheim
https://rheingaulinie.de/files/rheingaulinie/files/Freizeit/KunstKultur/Niederwalddenkmal_Ruedesheim/Seilbahn/P7051941.JPG
Concentration camps: If you'll be in Munich, then Dachau makes sense. The bigger question to me is whether visiting any concentration camp is a wise decision. I believe these camps were turned into visitor centers mainly for post-war German citizens (and future Germans) who were/are obligated to see exactly what Nazi Germany was about. I'm sure they are also important as proof to Holocaust deniers that they are horribly mistaken. The details on display there are gut-wrenching, but I seriously question whether the average visitor who learned about these camps in school will gain newfound understanding from a visit. Concentration camps are not tourist attractions and usually do not mix well into a day that's full of sightseeing and fun.