Set up a summer-long experiment to see how small changes in environment affect plants. Begin by planting identical plant seeds or young plants in 5 different containers (you can use cut off milk cartons or plastic bottles as your pots) and decide different watering schedules or quantities for each plant. Once a week, measure the growth of each plant. In which conditions did the plants grow best? How about worst? Want to try more? Repeat the experiment with the optimal water level in varying light levels. (Shade, partial sun, full sun, indoors.)
Extra credit: grow a plant that's native to your area in one pot next to a similarly sized plant that's native to a more humid or warmer environment and/or another that's native to a dryer or colder environment, all of which prefer the same light levels. Don't water any of the plants and see the differences in how each plant struggles or flourishes.