Salzburg to Dachau

Depends on what you are really looking for. Dachau is ok if you want to see a concentration camp, but if you want to be in a more pleasant environment, there is a lot of beautiful landscape around Salzburg, e.g. the Salzkammergut lakes, Hallstatt, the Bavarian lakes and castles, or even Linz with its remarkable Ars Electronica Center (museum of the future).

If you're into WWII places, you have the Mauthausen work camp just east of Linz that can be reached by train. It's east of Salzburg.

Dachau seems like a long day trip to me, but it's your vacation, of course, and you should do what you want to do.

We can probably provide better advice if you tell us when you will be in Salzburg, what you already have planned, and what you like to do.

Dachau is a really long way from Salzburg. I don’t see these two places linked as a day trip. Best case scenario, it's like a 4.5 hour round trip?? Likewise, Matthausen is almost as inconvenient from Salzburg.

There is the very worthwhile Document Center Museum right in Berchtesgaden, but check to see if they are open. They’ve been undergoing a big renovation and have been closed this past Summer.
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Other options:
Jennerbahn gondola/Königsee boat ride with some easy hiking

Eagles Nest/Berghof (seasonal)

Salt mine tour in Berchtesgaden

Werfen ice cave