Any ideas for my science class?

You can ask this question in the Homework Help category & probably get a lot more answers.

Grow some quick-growing potted plants, otherwise identical, under varying conditions: more or less sunlight per day, more or less water per day, different types of soil and/or fertilizer,etc..

At the end of the trial period, measure their height, amount of foliage, etc. to see how the controlled variables affected plant growth.