The High Line is an elevated urban garden that winds through Manhattan's West Side, following the river from Hudson Yards in the North, through Chelsea to Greenwich Village in the South. It's free to visit, and is well worth doing.
It's a welcome break from the hustle and bustle of Manhattan, and a little oasis of calm amongst the bedlam. For us, it doubled as an easy walk from Hudson Yards to Little Island, where we stopped for a coffee before exploring Greenwich Village.
Better than the gardens, for me at least, were the views. The trail is built on an old elevated New York Central Railroad line on the West Side, and snakes easily between several different architectural eras, from the shining glass facades of Hudson Yards to the red brick warehouses of the Meat Packing District.
There are tons of fascinating buildings to see, and great views across Manhattan down the long parallel streets of Midtown. I'd recommend joining the High Line at the elevator on West 30th Street, and then walk South to Chelsea Market, where you can hop off and get something to eat.