A lot of the ideas that have been pitched in are very good, depending on what you are able to obtain, it is possible to grow crystals on many different objects, such as coal, brickes, strings, etc. You could also, depending on the type of experiment that is needed to be done, do somethong human realated. For example, see which gender, boy or girl, reacts morre to different types of scents, like cheese, spoiled milk, perfume, fish, etc. It mostly depends on what age/grade you are in.
