The dreaded layover has long been a source of travel frustration: Inconvenient connection times leave you with an empty chunk of time, and if the travel gods have routed you through a small city with few attractions, you have little choice but to waste precious vacation hours watching TV at an airport bar or wandering the terminal. If, however, you transit through a major hub like Abu Dhabi, Hong Kong, Singapore, Dubai or Doha, rejoice! These blossoming metropolises are not just easy to get to f...