Walking around Berlin with your camera in the hand and no inspiration? Don’t forget the underground stations! They have many advantages: there are plenty, so you can choose your preferred, they are always there, so you don’t risk to miss the moment, they are often crowded with all sort of people coming and going, so they are easily a must for a street photographer and they offer many cues for a nice architectonic shot too.
The Berlin U-Bahn is one of the most extensive underground network in Europe with more than 170 stations spread across ten lines, so you really can choose your preferred one to take some nice pictures. Have fun!