The first day I would build
Start with the view that explains the city, then let the streets get louder.
- Begin at Kalemegdan before the day gets too hot or busy.
- Use Knez Mihailova as the central spine, not the whole plan.
- Let Dorcol or Skadarlija soften the late afternoon.
Kalemegdan is the right first anchor because the rivers, fortress walls, and city layers make Belgrade easier to read. After that, Knez Mihailova gives you the obvious central thread without needing much navigation.
I would avoid turning the first day into a museum-like checklist. Belgrade is better when one formal sight is followed by coffee, a long walk, and an evening that can stretch without feeling scheduled to death.