What is a good science project idea?

I am in lower Key Stage 3, and I can buy special materials, but not too many or too much money!! I am a level 5a to 6c, anything that level?? If you have any ideas please post them here as a reply, even if you think it will be too hard, I have 2 months to do it!! xx Oh and it has to have a model in it and some facts xx

You could demonstrate how to determine the best amount of fertilizer for a particular plant by placing seeds in the ground, one group with no fertilizer and the rest with increasing amounts in small increments. Mark the amount each one got. Once they are growing you can observe which grew the most and the point where too much was applied and the plants suffer from fertilizer burn or don't grow at all. Then photograph the results for your report.

The above can also be done by adding controlled amounts of water to plants to determine how much produces the ideal growth and how much is too much.