The first route I would choose
Let Kalemegdan explain the city before you chase more pins.
- Start with Kalemegdan and the river view.
- Walk Knez Mihailova back toward Republic Square.
- Use Dorcol for coffee or a slower afternoon.
Kalemegdan is the best first anchor because it gives Belgrade scale, history, and the meeting of the Sava and Danube in one place. After that, Knez Mihailova is the natural central spine.
Dorcol makes a good second layer because it shifts the day from formal sightseeing into everyday Belgrade: coffee, side streets, older buildings, and a rougher charm.