I think Belgrade is one of those cities where you really need either local friends to show you around the edgy happening bars, or a guide to show the historical significance of the Habsburg, Ottoman and Soviet legacies that aren’t brought to life as well as they could be.
Belgrade Fortress is the obvious site worth checking out for its views over the Sava and Danube rivers meeting, but outside of that I don’t really remember that much from Belgrade and it feels quite low down the list of highlights of the Balkan States.
If you are though keen on a trip around this part of the world for its stunning castles and old towns, and some of the best coastline in the world all compactly housed within 8 countries, see this travel entry for an itinerary and general tips for how to get the best out of your trip – 3 week trip around the Balkans States.