heres a good idea for you...
i did this back in grade 9 and got a really good grade.
the effects of different liquids on plants.
i had about 10 samples (10 small plants in individual small pots) all available to show off...
each plant had been fed using a different liquid... water, tea, coffee, milk, orange juice, coke, etc...
i did the experiment over a 1 month cycle and displayed all of the plants at the end and a write up on each, for every week.
it was pretty bad ass.
my only tip is to really make your project look top notch... you can have the best idea but a shitty display and you'll fail. make it look good and thats half the grade.