The fantastic ruins on Kilwa Kisiwani which means "Kilwa of the Island", a UNESCO world heritage site (http://whc.unesco.org/en/...). Most hotels will provide you with a boat to the island and a guide. Guides with detailed knowledge of the history and archaeology are sometimes hard to find but they ARE around, but there are extremely informative signs to help you out if your guide only knows the location of each of the ruins and not the story behind them. There is also a useful book available with the history of the island and a detailed description of the ruins, which makes a good guide and a nice keepsake. See the Wikipedia page for more information.
The town of Kilwa Kivinji is less popular than the other two Kilwas but it is a charming, sleepy Swahili town, much of it built in the traditional style out of traditional coral-rag. It doesn't have the 'sights' of Kilwa Kisiwani but it is a pleasant town in it's own right and well worth a day trip from Kilwa Masoko.
