For this first day on the island of Pemba, I suggest you take the opportunity to visit Chake Chake which is the main town of Pemba. It is a good starting point for visiting the island.
Stroll around the city and you can in particular visit the Pemba museum which will teach you a lot about its history.
The island is very famous for its clove plantations, so go visit a clove oil distillery.
In the vicinity of Chake Chake, go visit the ruins which are nearby (Mkame Ndume or Ras Mkumbuu).