Another active snorkeling or diving day or a day at leisure in the accommodation of your choice.
Depending on the departure time of your flight it might be necessary to return to Stone Town for overnight. You can choose to leave in the morning and spend a half day at Prison Island or if you have missed the sunset cruise book it for the late afternoon at Stone Town.
Alternatively you can stay at Mtoni Marine Centre which is on the waterfront North of Stone Town (15 min). Another option is Mbweni Ruins Hotel South of Stone Town (10 min).