Two stops, one boat trip. You head out from Stone Town to Prison Island first, where you walk the grounds, see the giant Aldabra tortoises, and learn about the island's history as a former prison and quarantine station. After spending time there, the boat takes you to Nakupenda Sandbank.
At Nakupenda, you swim, snorkel, and relax on the sand while a seafood lunch is set up, grilled fish, octopus, prawns, and fresh tropical fruits. Both spots are a short boat ride from Stone Town, so there's no rushing between them. It's a solid half-day on the water that covers history and beach time in one go.