In choosing a science fair project, try to pick something that you have a basic understanding about. You want to test the relationship between two factors, known in scientific terms as the independent variable and the dependent variable. You basically think the availability of one variable influences the other, such as the amount of sunlight plants receive has an impact on how much they grow.
Other ideas worthy of consideration would be the effect of temperature on water dissolving sugar; the effect of mass on force generated; the effect of acceleration on force generated; the effect of density on bouyancy. All these ideas demonstrate the relationship between the two factors called the independent variable and the dependent variable. Virtually anything that you think causes something else is testable in the form of an experiment and would make a great science fair project.