According to AD100 architect Bjarke Ingels, going green isn’t just a serious business—it can also be fun. In Copenhagen, the Danish wunderkind and his firm, BIG, recently completed a waste-treatment facility that converts trash into energy while also luring thrill-seekers. Its 107,640-square-foot green roof doubles as a hiking trail and year-round ski slope; a façade reveals a climbing wall; and two bars provide rooftop or après-ski libations. “The project is a crystal-clear example of hedonistic sustainability, the idea that a sustainable city is not only better for the environment, it is also more enjoyable for the lives of its citizens,” says Ingels, who sees addressing the climate crisis as a design challenge, not simply a moral imperative. Though the plant does important work for its city, producing enough clean energy annually to power 150,000 local homes, it is also a public amenity, injecting both adventure and curiosity into the Danish capital. Says Ingels: “It makes me excited to see what ideas this summit may spark.”