While it might lack the world-class cultural institutions of Berlin, Belgrade, at the confluence of the Sava and Danube rivers, offers its own distinctive appeal, with storybook historical districts and venerable fortresses and parks. Better still, this center of Slavic cool just got much easier to reach: Direct flights from North America, via New York City, started in June on Air Serbia. Throw in Belgrade’s cafe scene, low prices, talented young designers, emerging former industrial districts, sausage-filled cuisine and unbridled night life — from D.J. bars to party boats to all-hours nightclubs — and the comparison seems even more compelling.