Kilimanjaro usually works best at the beginning of a Tanzania trip, before safari or Zanzibar. The climb demands energy, focus and acclimatization, while safari and beach time work naturally as recovery afterwards.
Most travellers should climb Kilimanjaro before safari or Zanzibar. The mountain is the most physically demanding part of the trip, so it makes sense to do it while your energy is highest.
After the climb, a Northern Circuit safari can feel like a reward — Tarangire, Ngorongoro, Serengeti or Arusha National Park depending on time. Zanzibar can also be added at the end for beach recovery, especially after a longer trek.
The main planning decision is how many days to give the climb. A slightly longer route often creates a better experience than rushing the mountain.