We’re traveling to Munich Oct 1-6, taking the train to Salzburg for a few days before driving back to Munich through the Garmisch area. We’re already planning visits to Dachau, The Eagles Nest and Neuschwanstein but looking for suggestions of day trips from Munich and great places to visit along the drive. This is our first time in Germany
Did you already have searched the forum for earlier similar questions? A lot of options are already waiting for a search.
Example: https://community.ricksteves.com/travel-forum/germany/hidden-gems-in-munich
Also Munich's website provide some tips for tourists.
Including a possible overnight or two outside Salzburg and Munich at/near your stated route/destinations seems most of your time is spent.
Look at this YouTube channel: https://www.youtube.com/@NearFromHome
Scroll down and find the Munich Day Trips section. You can actually see the options!
Have you been to the Tegernsee 50 km south of Munich?
Car from Austria or Germany ? if cross border rental returns expect high charges.
Austrian into Germany, Munich and others require a green umweltplakette to drive into the city. Cost and hassle.
German into Austria requires usually toll Vignette of around 10€ for a short stay.
Have you got hotels yet? You will be at the end of Oktoberfest and rates are high and rooms scarce.
Check out Passau.