The Mnarani aquarium is a little walk away from the traditional village of Nungwi and, while the place itself is not spectacular, your visit will help the preservation of sea turtles.
Injured turtles are nursed here at the aquarium - and they also have a reprduction project. For the 2 dollars entrance fee you can feed them seaweed and even swim with them, although the water is a little murky. You may also lift one (the young "ranger" will encourage you to do so, and show you the correct way so as not to discomfort the turtles) to see how heavy these creatures are.
Kendwa is about 3 kilometres south of Nungwi, and it's easily reachable on foot via the beach - however only at low tide.
When you there there you see first a posh all-inclusive 5 star hotel (la rosa dei venti) with a wonderful beach and a restaurant built on stilts, and then another great beach with simpler accommodation and a traditional village.
It's also possible to get there by boat from Nungwi, and it shouldn't be too difficult to find a fisherman ready to take you there.