This giant, youthful activity space (let's call it) is a wide-open, out-of-service airport lot. Allowed to just gradually return to nature and recreational use by citizens, it's a convincing example of how effortless yet effective urban renewal can be. Mileage markings along the paved runways and occasional signage are the only guides to the park's use-the rest is up to the citizens, or traveler's, imagination. Many Berliners grab a beer from a local Späti and gather here to watch the sun set across the expanse.