Hiring a bike in Berlin is mostly quite expensive, on average €12 per day, so as the weather improved I considered buying a second hand bike and selling it again in August when I leave, however many people told me that bikes were constantly being stolen despite being locked. Then I saw some people riding Lidl-Bikes. I don't know why I didn't notice them before because they're all over the place. The best value is to pay a monthly charge of €9 and then 30 minute cycle trips are free if you return the bike to within 25 metres of a U-Bahn station, and other places, but you can leave them anywhere and pick them up anywhere using the map in the Lidl-Bike app. If you don't leave it at a drop zone by a u-bahn it costs 50 cents.
The tricky part was getting registered on the German-only website. I used the Chrome browser which translates every page to English but I still had to call their support line as the SMS activation code wasn't sent to a non-German mobile. Anyway once I got started I went for a Lidl cycle all around Berlin today - three times further than I could have walked - the freedom of the city was mine, for €9.