Berlin is a sprawling city, which makes exploring exclusively on foot a considerable challenge. If long walks or jaunts underground don't appeal to you, test out the city's bike-share program, Call A Bike. It looks similar to Paris's beloved Vélib but requires a mobile phone.
You call the number printed on the side of the bike to register and obtain the code to unlock it. The cost is €0.80/minute, up to a max of €15 in a 24-hour period (payment by credit card only). You can also rent bikes through private structures like Fat Tire Bike Company, just be sure to follow the rules (more info here).