I stayed 5 weeks at the Mnarani Aquarium in Nungwi.
The time I spend there was amazing, the work there is pretty simple but you still will learn something new about the turtles there. The local staff is always friendly to you and you can make jokes with them. They will treat you like Family everyone belongs to this family.
The other volentuers are open minded and are coming all over the world so you get many different cultures there.
And on the weekend you can explore the island by your self and you have enough time.