The northernmost point on the island of Manhattan is in Inwood Park, just off the trail, and you can get there by scrambling down some old stone steps. The easternmost point on Manhattan is right about where West 220th would meet the Harlem River and you can get there by ducking through a hole in a chainlink fence. The points are less than 2 miles from each other, if you simply can't choose and you want to nab them both.