"Oh my God, this is going to be amazing," a preadolescent wearing a gray hoodie exclaimed as he dashed in to The Yard, a 50,000-square-foot adventure playground on Governors Island in New York Harbor. The Yard, for kids 6 through 13, lacks the usual monkey bars, slides and swings. It is, however, well-stocked with dismembered store mannequins, wooden packing crates, tires, mattresses, an old piano and assorted other detritus of the modern world.