Hi Charley, first you should check out some of the other busking threads here on Mudcat. Put "busking" into the search box and a fair number of threads should come up.
In Bonn you need a permit. I can't remember which office it is you have to go to. However, there are small towns around like Siegburg, Troisdorf, Brühl and others where you are more likely to make money - especially if you get there before midday on a market day. There is always a trade off between volume level and being agreeable to other street users - including other buskers. If you are loud, more people are likely to hear you. On the other hand, there are more likely to be shop owners etc who won't want you around for very long!
You do not need a permit in most German towns - certainly in NRW. Whatever the law says, the bottom line is still the same. If a shop keeper requests a policeman to tell you to go, the policeman has to do it. I do not know of any country where this isn't true. Mind you, I have only ever played in England, Germany, Holland, Belgium, Luxembourg, Switzerland, Austria, Italy, Sweden, Denmark and Czechoslovakia. It may be differeent elsewhere!
Good luck!