Breakfast down at Clintonstreet Bakery or Pastrami in Katz Deli (Reuben Sandwich = superb)
Walk over Brooklyn Bridge from Brooklyn side to Manhattan, short walk to WorldTrade. Or Pick up the water taxi that goes around the Statue of Liberty and drops you off right by the start of the Highline walk, good pizza at Artichokes about halfway along the route!
Pick up a hire bike and do the 6mile loop of CentralPark, Walk down Fifth Avenue. Easy to pop into Rockefeller or Empire State if you want a rooftop view.
If you already have a subway day pass get it back to the airport! Much, much cheaper than a taxi!!