"Road less traveled" from Ga-Pa to Munich?

Youn could take the B2 direction Mittenwald and turn north at Krün to lakes Walchensee and Kochelsee. At Urfeld on the north end of the Walchensee there is a cable car up to the Herzogstand. If you bring sturdy shoes you could walk over the small ridge to the neighbouring Heimgarten; there are marvelous views on both lakes and on the Tyrolian Alps. There are mountain huts on both peaks which offer simple meals.
In Kochel there is the interesting Franz Marc Museum. 10 km to the north there is the baroque Abbey of Benediktbeuren (where the famous Carmina Burana were found). Some 15 km west of the Kochelsee there is the beautiful and quiet lake Staffelsee which offers a lot of nice and effortless walks.

I would definitely second SLA's suggestion of the route along Walchensee. I've done that trip between Ga-Pa and Bad Tölz twice by bus, and it is very scenic, at least up to Kochel.

The trip you are planning is very doable by public transportation. A Regio-Ticket Werdenfels (20€ first person plus 6€ extra persons up to 5 total) will cover both the train to Ga-Pa, then bus RVO 9608 to Kochel, and by bus and/or train back to Munich. Here is a map of the covered lines.

The route is scenic. It would be a shame for one person to miss all that scenery because they are driving and watching the road (particularly the switchbacks on the road coming down the mountain into Kochel).

Another vote for driving via Walchensee and Kochelsee. This is the route my family would take when we lived in Munich and were headed to Garmisch Partenkirchen. There's a parking lot in Kochel am See just below the Franz Marc Museum where we often stopped for a picnic by the lake.

DJ

speaking of parking lots, if it is a nice day expect the A95 to be one. I've seen delays of over an hour reported on it.

Thank you all for the responses---we will definitely look into those suggestions further! (Looking at the map "cold", we were leaning toward that route with the lakes but then you never know what you are going to get from a scenery perspective)

Just a point of clarification on the car: we are very comfortable with taking trains/local public transport but the drive is actually part of the "adventure" in this case. My husband really enjoys driving so we are going to rent an "impractical" vehicle from BMW On Demand for the day and build that into the memories.

Good point about the A95, we'll certainly check traffic on the GPS and take other routes if needed.

I expect you may know already, but despite the large number of former US soldiers in the region what you refer to as Ga-Pa is actually known as Garmisch-Partenkirchen.