Most travelers finish on the beach. After days of early game drives, or a Kilimanjaro climb, a quiet coast is the reward.
Finish on the beach
Best for most trips, and essential after a climb. Fly from Arusha or Kilimanjaro to Zanzibar and rest.
Start on the beach
Useful if you want to recover from a long flight before early mornings, or if you are on honeymoon and want the romantic part first. If you are climbing Kilimanjaro, arrive in Arusha early to acclimatize instead.
Both
A few nights at the start and a longer stay at the end suits longer trips.