Central park, Battery Park, Prospect Park, Washington Square Park, Madison Square park, Morningside Park...Who said New York is not green enough?
There is always a way to escape from the concrete come on!
A nice and brand new spot (means not too crowded yet) is definitely Dumbo.
Don't know where it is? Very simple DUMBO = Down Under Manhattan Bridge Overpass. Just follow your legs down to the bridge, see this little green hill, here you are!
Bring some friends, a picnic, a guitar or two and let's jam dreaming in front of this amazing view of downtown Manhattan. If you are courageous enough, you can always go have a look at the Ice Cream store down there or even go explore the neighborhood walking on the Brooklyn Heights Promenade, taking pictures of the cute little houses in the fruity area (cranberry, orange, pineapple street) or go up to Vinegar Hill this little haven of peace in the middle of nowhere...
Exploration is the best!