For your last full day in Zanzibar, the Stone Town visit is a must. Indeed, Stone Town is the "capital" of the Zanzibar archipelago. It is a city rich in history, culture and traditions that I can only recommend that you visit at least half a day.
For more information on what to see, I recommend you read this article much more complete on the subject.
For the second half of the day you can board a boat and go on Prison Island which is located 5km from Stone Town. This island is also culturally and historically rich and today it is home to a conservation reserve for giant tortoises. There you can visit the island, learn more, see the turtles and swim. For more details, I advise you this article.