Here's some ideas from Labs that I did in Grade 9 Bio, Chem and Phys.
1. Mouldy Jello. Make Jello, and some old (and mouldy) bread or cheese. Get some mould on the Jello. The sugar and the protein in the Jello will help it grow. You can do different concentrations of Jello.
2. Corrosion. Put different metals in acid. Mass them before, and then mass them every day (take them out of the acid with tweezers and place on petri dish, then mass) to figure out how much mass was lost because of corrosion. Do some research and make sure you use a corrosive metal, a noncorrosive metal, and one in between.
I hope this helps!
Grade 9 Science course.