The first day I would build
Give the city one clear route before adding extras.
- Start with Alexander Nevsky Cathedral and Serdika Roman ruins while energy is high.
- Use Vitosha Boulevard as the natural reset instead of crossing town too early.
the easier plan is Alexander Nevsky and Serdika first, Vitosha Boulevard for the soft landing, Boyana or Vitosha as a separate block. That keeps the day readable instead of turning every good name into a separate detour.
I would rather leave one place for tomorrow than drag a tired route through Boyana Church just because it looked close on a map.