Example of Investigatory project in science for grade six student?

Everyone does the bean-growing thing, but people can get a bit more creative with that. For instance, leave some of the leaves alone and cover another to show that the others will be green but the covered one won't.

Or leave one bean stalk in a dark room with just red artificial light and another with green (you could put a cheap filter which polarizes the light for this.

The one in red light would probably grow better.