Haha, these are great! I especially like "Dumm wie Brot." I'm not sure if it counts as an idiom, but I often hear people say "Kopfkino." As I understand it, it means the imagery of something bad or worrisome which plays in your head. Another idiom to add to the list is "Ich verstehe nur Bahnhof." I haven't actually heard anyone say this to me, which is a good thing I think, but I read it once before coming to Germany and it has stayed in my head. It is when the speaker doesn't understand what you are saying, so they say, "I only understand train station."