Changu Island, also known as Prison Island, is located about a 30-minute boat ride from Stone Town, and its clear turquoise waters make it hard to resist snorkeling. A visit also allows you to understand a bleak period in Zanzibar's history. This small island was used as a prison for slaves, and by 1893 the actual prison complex had been completed, but instead of housing prisoners, it was used to quarantine yellow fever cases. Nowadays, the island is home to some endangered Aldabra giant tortoises that you can enjoy watching up close.