Jozani Chwaka Bay National Park, in the centre of the island, holds the last large patch of native forest on Zanzibar and the only population of Zanzibar red colobus monkeys in the world. The troops near the visitor centre are habituated and easy to photograph on a short guided walk, which also takes in a raised boardwalk through a tidal mangrove forest. It's a genuine change of pace from the beach and works well combined with a stop at a southern-coast beach like Paje on the way back.
Pick-up from Stone Town or your beach hotel. Guided forest walk among the red colobus and the mangrove boardwalk, with an optional beach stop on the way back.