Hydroponics is the practice of growing plants in water instead of soil. Many plants can be grown successfully in this manner. If you're interested in hydroponics, you might want to do an experiment in which you test whether bean plants (or another type of plant, if you prefer) grow better in soil or in water.
You could start with seeds or plants of the same size, planting one in soil and keeping the other in water, while making sure other factors such as heat and light are the same.
Hopefully, you've gotten an idea of what an interesting area of science botany is, and will think about possibilities within the field for future science fair projects.