Germany: Don'ts

It is not common to use the car horn frequently.

Don't smoke in restaurants, bars, pubs, in public buildings etc.  There are relatively strict laws and regulations concerning smoking. In public places such as airports or stations there are designated smoking points where people can smoke.
Camping is not allowed outside camping places.
Don't talk to other guys when using the toilet.
Two guys don't kiss when meeting each other.

Don't park where it is forbidden.  Otherwise you can easily be fined or even towed away.