I liked testing various ways to increase plant growth, such as changing the amount of N, P, and K in the fertilizer, or using growth hormones. To do it right, choose one plant type, say, corn or soybean.
One of my favorites is to create a bottle rocket that is air powered. Use a soda bottle, a cork, and a bicycle pump. When air is pumped into the bottle, it can travel pretty high (I've seen these crash into the ceiling inside a gym, so be careful where you aim it). Then you could try different sized bottles and compare the height (as a guess more than an actual reading) based on the amount of air that can be pumped inside the bottle. I have seen this done with water, but it is very messy and doesn't always work. I attached a website below for a simple air powered bottle rocket. Enjoy!