7th grade science fair project ideas are likely more involved than the projects you've done in previous years. The science fair at your school is likely more competitive for your age group and there's a good chance your peers will be putting a lot more time and effort into their projects than they have in previous years, so you'll want to do the same for your 7th grade science projects.
One interesting idea might be to see which type of cheese mold will grow on the fastest. This is a great project because it's interesting, but still fairly simple. Another idea for a project might be to see if you can grow a plant without soil. You can try placing seeds in various things such as a wet paper towel, grass clippings, coffee grounds, etc to see if they will grow. Be creative!
You could find out whether the presence of smoke in the air affects plant transpiration. Perhaps you could look into whether or not there is ever acid snow, we all know there is acid rain, but what about snow? You can find this out by testing the PH level of the snow and comparing it to the PH level of acid rain. Obviously this particular project only works in the winter months. There are many great topics out there for your 9th grade science project, it's just a matter of finding one that interests you, researching and testing it out and presenting it in a professional manner.