For an international traveler, Zagreb is the perfect place to arrive, and then make connections to other Balkan nations - whether you are road tripping or traveling by public transportation.  Zagreb is also great for budget-conscious travelers as you can easily find cool accommodations, free attractions, and affordable eats in the city.  Both Zagreb and Ljubljana are not overly crowded (like Paris or London), and their city centres are very compact and easily accessible on foot. Ljubljana surely has a charming vibe with the river flowing through the city.