The place
Zanzibar, in short.
Zanzibar is not one place. Stone Town is a dense coral-stone maze of Omani, Indian and Swahili building that has been trading for a millennium and is a UNESCO site. The north is beach resort country. The east is a long line of villages, seaweed farms and kitesurfers. The interior is spice smallholdings and one remaining patch of real forest.
The thing to understand before you book is the tide. On the east coast the sea withdraws close to a kilometre at low water and comes back over hot sand. It is beautiful and it is not a swimming pool. On the north coast at Nungwi and Kendwa the water is deep enough to swim at any state of the tide, which is why the north is busier and costs more.
It is also a Muslim island, and outside the resorts that matters: covered shoulders and knees away from the beach, and a different rhythm during Ramadan.
Most people come after a safari, and four to seven nights is right. Two nights in Stone Town and the rest on a beach is the pattern that works.