Okay, since the other experiment had too many variables, i've decided (after researching) that this one is a good idea.
Identifying Liquids In Which Seeds Grow Best.
From this site: http://www.sciencenerddepot.com/Easy_Science_Projects.html
Basically, I have to place bean seeds into cups with different liquids in them (water, water with dissolved plant food, soft drinks, juices, etc.) and record their growth over regular periods of time.