Start with a familiar plant like a tomato. Allow students to manitpulate most important factors such as sunlight, water availability, and soil compostion to showcase how each plays an important role in plant growth. With the right combination of factors, the plant will grow rapidly and healthy, whereas if the student decides to starve the plant of water, the plant will wilt or even perish. In this way the student can actively be engaged in learning, by having direct control of how well the plant grows. Could even create a small competition between classmates to see who can grow the healthiest plant with the least amount of resources used.