Testing flammability in children's clothing.
I got kids clothes that had the low fire danger tag on it, cut them up into equal pieces, lit them one at a time and timed them. Repeat. At least 3 times.
Variables were the different materials. Probably best to get quite a diverse range if possible. I included other observations like if the material turned to ash or if it melted into a black goo.
It was pretty cool, and satisfied my inner pyromanic.
Got 49/50 which was good. And the only reason I lost the mark was because of my bibliography.
Others, that my friends did and got good marks for were:
-Amount of iron in cereal: crush the cereal to a fine powder and then use a magnet to collect iron and then measure on scientific scales.
-Carbon dioxide in plants: to be honest, I don't know how she did this. All I remember is that lots of people did it, and it involved growing things and limewater.
Hope that gave you some ideas
EDIT: Oh, I forgot to mention, it's less about the idea and more about how you conduct and structure your report. I mean, the idea is important but, also think about how you are going to structure everything around it i.e photos, processes, observation, accuracy, background information, relevance to society etc.