Inwood Hill Park is home to the real New York of old, back when the island of Manhattan was all trees. The peaceful, quiet, and very green park that featured wide, winding trails for walking is an oasis within the city and one of the nicest parks I've been to in the area. Hard to believe that busy highways and giant hamburgers (served on the rare side at the sports bar circa 1970 Piper's Kilt around the corner) surround the serenity of these huge and ancient trees.
We only explored a portion of the paths, which immediately give you the sense that you'll become lost forever with the myriad splitting of trails and lack of any signs, but really, it ends up being pretty easy to navigate so long as you keep the river (with lovely views of the Bronx) and the city of Inwood as your guides.
It's Manhattan's second largest park next to Central, but fairly unknown and forgotten. Use that to your advantage and enjoy secluded walks in the shade of nature this summer.