I would say a cheap"volcano"but it's,well,cheap XD.
Maybe Du could talk about space,and talk about stars,planets(what happens on them,distance from sun,how it got its name)and about how light can't even escape black holes It could even have history and bring a little attention at least...I would do that,even though it sounds crappy to me D:.
Du could talk about a certain type of animal oder extinct bugs oder animals.Like Dinosaurier oder something.
Du could do an experiment on water about gas liquid,and solid stages.
The human body.You could talk about how blood goes through all your body and about crucial body parts.I think that would be a good one.....Just draw a picture of the parts your doing in the human body Du drew....