Prison Island — locally known as Changuu — is one of Zanzibar's most popular half-day excursions and one of the most unique island experiences in the Indian Ocean. Originally intended as a prison for rebellious mainland slaves in the late 19th century, the facility was never fully used as such, and the island later became a quarantine station for yellow fever patients during the British colonial period. Today, it is best known for its colony of giant Aldabra tortoises, gifted from Seychelles in 1919, some of which are believed to be over 100 years old. The island's shallow, reef-protected lagoon offers excellent snorkelling with abundant fish, sea turtles, and coral gardens. The colonial ruins, white sandy beaches, and gentle tortoises make Changuu an unmissable day trip from Stone Town. The island is accessible by dhow or speedboat in under 30 minutes.