I had a great time at Karin's and Vural's place. It is located a bit north of the city center and can be easily reached by bus. I had very interesting conversations with Vural and Karin. As it turned out, they are both fluent in German.
I can especially recommend this place if you like animals. There are two cats, three dogs, countless turtles, and a chicken that share the garden among themselves.
The only downside I can see is the distance from the city center. It shouldn't take longer than 30 minutes on the bus to get there when the road is free, but traffic jams are very common in Istanbul, so better bring a book.