Great idea, thank you! As I mentioned in my post, I actually went with the intention to meet new people, but it was pretty empty. But the idea about being a native guest teacher is absolutely amazing! Next time I get the chance to have an extended stay of at least a couple of days, I'll try to remember to ask the local Goethe-Institut in advance whether they'd be interested.
But yes, events are fairly limited in summer. But to be fair, I only know of the public events of the institute, which are listed on their website. The librarian didn't tell me about any internal events they currently have.
Have you ever done guest-teaching in a Goethe-Institut or how did you come up with this wonderful idea?