Did you have at that time a US drivers license?
Thanks for info about the lake, will try to stop there on the way even though it is a bit out since we are aiming for Zell Am See.
Yes, I had a US license at the time. Maybe the laws changed? I don't think anyone will actually stop you and ask for it - if the car rental place said you don't need it, maybe you don't (although DROPR, as you can see what happened with me with the carseat). Did you ask them if that applies in Austria as well (because you will be taking the car to a different country)?
There were many things to do around Konigsee - you can google it. We came late in the day because of our carseat fiasco and we were traveling with a toddler so that limited us. We didn't go to Zell Am See so I can't compare to Konigsee, but it looks nice too.
Where are you planning to stay?