The High Line is this fantastic park. It runs from Ganesvoort St. in the Meatpacking District to 34th St. between 10th & 11th Ave. It was previously an elevated subway that transported food to restaurants. The story is that this particular stretch of track was the most dangerous track in the city with the most fatalities. So the track was abandoned and eventually transformed into a park.
You can access the park from Gansevoort St., 14th St., 16th St., 18th St., and 20th St. I would suggest just wandering around. The park boasts some of the best views of the Hudson and Hoboken on the West Side and is lined with benches that are perfect for reading in the sunlight. It is green and lush and there are pieces of art and amazing artists selling their stuff along the path. Check it out!