Alicia Keys sang about New York City as a “concrete jungle,” and in fact the skyline of the metropolis of 8.8 million people between the Hudson River and Rockaway Beach appears to be nothing but wall-to-wall skyscrapers. But the “Big Apple” is more than glass, asphalt and concrete: 14% of its area consists of greenery. There is of course Manhattan’s famous Central Park, but that’s just one of numerous green spaces, both large and small, that show a lesser-known side of the city. Admiring the skyline from a hammock Just an 80-minute ferry ride from Manhattan’s southern tip lies an oft-overlooke…