Just south of Ibo, Quirimba is the most economically active island of the archipelago, with large coconut plantations, a sizeable sisal factory and an airstrip. It's not particularly scenic, though it does have some lovely stretches of sand. You can walk (with a guide) between Quirimba and Ibo at low tide, a beautiful excursion incorporating dense mangroves, expansive beaches and warm tidal pools.
Quirimba was an important Muslim trading centre well before the arrival of the Portuguese. In 1522 it was raided by the Portuguese and the town was destroyed, but it was later rebuilt. In the 16th century Quirimba served as a centre for missionary work.