Three days in Germany with WWII and automotive focus

So besides scenery - which you will have a lot of on your river cruise - you're looking for suggestions in Germany that "are most highly recommended for a first timer" and that will tease out your "shared love of history" in the course of 3 days.

I take it you are flying into MUC.

Regensburg is Germany's most well-preserved medieval city. It's 1.5 hours by direct train from MUC. And Regensburg is 1 hour by train from Vilshofen. Seems like a reasonable destination if you want to maximize your available time and present a representative scope of the German history. I'd stay in Regensburg for the 3 nights. See the city on your first day for whatever amount of time you have.

https://www.regensburg.de/welterbe/en
https://www.regensburg.de/welterbe/en/visitor-centre/free-tours

On Day 2, take a day trip to Nuremberg. Great place with great museums for anyone who wants some insight into German history and culture. Since you've done a concentration camp - and if you're doing anything WW II in Munich - I would lighten up the dark WWII stuff and avoid those history lessons in Nuremberg. Lots of other things to choose from in this city...

DB Museum: https://dbmuseum.de/en/nuremberg/exhibitions/the-history-of-the-railway-in-germany

Tucher Mansion Museum: https://museums.nuernberg.de/museum-tucher-mansion/

Museum for Industrial Culture: https://museums.nuernberg.de/museum-for-industrial-culture

Kaiserburg: https://www.kaiserburg-nuernberg.de/englisch/tourist/index.htm

On Day 3, take a day trip to Munich and back. 1.5 hours by train. Sounds like you already have some plans for this place.

On Day 4, take the train to Vilshofen (1 hr.)

Berchtesgarten was very hard for us to find as there was so signage then. It had a very strong effect on us once we got to the top where we could see how paranoid Hitler was, escaping to this high mountain top where he could look out on his land.

how paranoid Hitler was, escaping to this high mountain top where he
could look out on his land.

I don't think this is an accurate reflection of the state of affairs. The Obersalzberg complex including Hitler's Berghof was created to have a Bavarian base of operations for the Nazis (RS calls it a "Camp David" for the Nazi regime) and to carefully craft the lore of Hitler as "Man of the Volk" in the 1930's.

Please identify clearly your interests because WWII is a typical wrongly mix-used tourist label:

• Holocaust (nothing to do with WWII)
• Nazi terror regime (repressions and take-over of power) and resistance
• WWII war related things like memorials and / or documentations

If you would start your journey in Berlin which I would recommend for the above mentioned then you can consider visiting BMW and / or Porsche in Leipzig before heading on to Vilshofen.

If you're going to Munich, you're not far from the Dachau Concentration Camp. It's well worth the visit. It's an eye opening and soul wrenching experience. There's a BMW Museum there that should be worth a visit. Haven't been to that museum, but can definitely recommend the Mercedes-Benz Museum if you're able to make it to Stuttgart. Been there many times. In Berlin, there is a lot of history to see ... Checkpoint Charlie, the remaining sections of the Berlin Wall, The Topograghie of Terror, which was the Gestapo's HQ, and many more.

So three days...does that mean three nights and full sightseeing days? Think about your actual schedule and factor in schlepping to and from your hotel.

Where are you on day -1? If you're coming from the states, where does your flight land? If you're in Europe already, where are you?

That said...
Berlin is too far away
Nurburgring is too far, only has limited public hours and is pointless without a decent car.
Munich has WW2, especially Dachau, and auto factory tours. Munich is the easiest from a travel connection standpoint
Berchtesgaden has WW2 and mountains. It's the most picturesque option. It's kinda doable in a short time with good planning.
Nuremberg has WW2, a cool castle and the toy museum. It's the most "compact" and doable in a short time.

They're all gonna be different flavors. You'll have a good experience at any of them honestly. Just choose the flavor you want, but remember you only have 3 days. That's not a lot of time. You don't want to waste inordinate amounts of that precious time screwing around traveling between places.

Since it appears there is an interest in WWII, you should check out a site before you go. The name is thirdreichruins.com and it has many photos of some of the places you are going taken just after the war. Next to those photos are ones taken much more recently, and the contrast is very interesting.

On the WW II interest, I would suggest others sites other than those you're planning on seeing, ie, sites in this western region of Germany, such Koblenz (museum), Vossenack (military cemeteries), Rastatt. Remagen.

In the lower Rhine area far more related WW2 sites but you're not going up there.