The city of Lisbon, Portugal, ranked as the happiest destination in the world for travelers, according to a study from BookRetreats, which offers packaged wellness vacations. That was thanks to its more than 2,800 hours of annual sunshine, expansive green spaces, and healthy restaurants throughout. "Walkability is another of Lisbon's mood-boosters," company wrote in its study. "The city is compact and easy to navigate on foot, with panoramic viewpoints that reward slow exploration."