Gosh, that is simply an impossible question to answer - there are so many potential projects, and that question is not where your son or daughter should start the quest. The quest should begin with an interest in finding out the answer to something about nature. "How do turtle hatchlings know which way to swim in the ocean?" A good teacher will know how to tease the interests out and help the child formulate a question that can be investigated. Ninth graders should have already studied science for several years and should know something of the scope of it. The point of a student doing a project is for the student to engage in an investigation and come to some closure on the question.
how about a composting projects its easy u could get to it bysearching it on the website to its nice and easy AND IT TAKES TIME so be sure u start it earlylolol thats a mistake i made one time and i will never do it again.but this is a good project so just try it and see what happens