You could do worse than replicate the mentos+diet coke experiment (see video).

Basically, the gelatin and arabic gum in the mento sweet interact with the carbon dioxide. Asparatame contributes to the reaction and the the whole thing can erupt fantasically.

Try it with other types of mentos, other sweets or food stuffs, especially those containing gelatin and arabic gum. Record which combination (of shaking and sweets) produced the biggest mess).

Also, there are lots of science projects that could be based on grossology.

Also, something that people forget is that psychology is a science, too. And all of us are fascinated with what makes other people tick.

School projects in psychology for age 9-12.

Psychology projects sorted by difficulty here and here.

Another way to approach this is to do either a very weird science project, OR to do one which a dramatic potential to go wrong. And show and discuss why it's such a bad or pointless idea as a science project, or prone to going wrong. Ideas here.