The heart of Liberty City easter egg
In Happiness island, steal a helicopter from the helicopter tours. Fly it over the Statue of Liberty and jump out of it at the statue's feet. You should land on a square platform with doors at the center of each side. One of the doors has a plaque on both sides of it that reads, "No Hidden Content Here." The door does not open but you can walk through it. Climb the ladder inside it and when you reach the top look up. There is a gigantic beating heart held in place by chains.