The main reason to go to Changu Island (also known as Prison Island) is the system of fairly healthy coral reefs surrounding it. Coral shaped like brains, antlers, trees, or boulders grows all around the island. The dead coral is a chalky white, but live coral comes in as many colors as it does shapes: brilliant yellow, deep red-orange, rich browns, and a forest of pinks. The fish in the coral can be gorgeous: giant starfish covered with markings like copper circuits, beaked parrot fish, and shoals of tiny, shimmering, brilliant blue fish glistening like sequins in the sunlit water. (This did vary, depending on where we swam; our first stopping point by the island had spectacular coral growths but fewer fish.) The only problem we had was that the water was actually rather cold; we were forced by the temperature to get out sooner than we would have liked. (Baridi means cold.)
The trip to Changu Island is a pleasure in itself. We hired a boat to get there ( $70), and found the trip -- through the blue water and the blue sky, leaving behind the picturesque Stone Town harbor, heading towards the smaller islands -- exhilarating. The boatman, Jojo, was friendly and helpful; he spoke little English, and we spoke even less Swahili, but goodwill made communication surprisingly easy.
We went on the trip for the snorkeling, but the island is also tremendous fun to explore. When we reached it, Jojo led us off the boat, along a sandy spit of beach, and up some wooden stairs set into the hillside. The stairs passed the beginnings of a small luxury hotel which is being built on the island. (Apparently all the construction workers come from the mainland, where people are willing to be hired for lower wages.) When the hotel is complete, it will have a wonderful setting; here's hoping it doesn't take so many guests that the island's sense of privacy isn't spoiled.
Past the construction site comes the island's other attraction: a colony of giant tortoises. For about $0.50, we bought big plates of spinach, which we fed to the tortoises; huge creatures with sharp beaks, which happily chomped up the spinach from our hands. Some of the tortoises let us touch them; their necks are dusty/dry to the touch, very muscular under a thin layer of papery skin. Jojo showed us a tiny cement tank containing some three-month-old baby tortoises, about the size of small turtles, and a larger tank containing some adolescent tortoises which had reached a foot in length; apparently the young tortoises are kept in these enclosures to keep them safe from rats.
After we were done feeding tortoises and snorkeling, we walked on the beach for a while, chasing near-invisible crabs at the waterline and watching a distant gray heron. Behind us, two boatloads of girls in Muslim garb arrived and began playing on the shore, wading into the water and collecting shells. We left happy.