I was in town to visit Berlin for the week and came across Rad Der Stadt Fahrräder. I rented two…read morebikes (12€ each/per day) and had a wonderful experience. You are allowed to keep the bikes the entire day you rent them, plus into anytime the next day - if you rent them early the first day and return them late the second day, that is nearly two days of a bike rental for the price of one! Great deal. Staff was very kind and friendly. My bike lock broke (my fault, I crashed and broke the key) and they happily took the bike back, offered a replacement bike, and did not charge me extra. Highly recommend for bike rentals!!