Normally I'm the one giving the advice here but I thought I'd try the wisdom of the masses this time!
I tentatively have plans for Tegernsee on Sept 18th and then in Munich on Oct 2-3 (Oktoberfest trip with my brother). I already have my hotel for Munich booked but that is it. I meet up with my brother in Frankfurt Sept 27th and travel with him back to Munich via a few nights in Franconia.
That gives me Sept 18-26 open. Right now my tentative plan is to get a holiday rental in Mittenwald and spend that week doing day hikes (maybe a night in a hut or two) but I'd be open to other suggestions for off the beaten path activities. I used to live in Munich so I'm familiar with all the standard tourist activities.
TIA
DJ