Berlin is one of those cities that has great groups and walking tours. I have done both private and group there and have found the basic walking group tour for 15 per person one of the best tours I have ever taken. I've taken it twice with two different tour guides and it is a wonderful city overview. There is a wonderful tour - wrong adjective- to the concentration camp. There is the outdoor pictoral museum- I will look up the name- that is a don't miss. It is free and gives the history of the rise of Hitler. It is a fascinating city.