I always loved coming up with science fair projects with my children while they were in elementary school. A good one for a third grader are playing two different kinds of music to two different plants to see which one grows faster and making a daily log regarding the plant and the music.
go to the site Google.com and ask good science fair projects
The back of your science books often has ideas you can use,
Your school website is a good website to find a science fair project. Science programs for kids typically have websites with projects listed. PBS has a website for kids that includes science projects.
google.com or science fair projects.com
Decomposition, energy, light, electricity, and photosynthesis are all good science fair topics
Good science fair projects could fall under a number of categories, including environmental science, astronomy, chemistry, and cooking/food science. For example, you could look at how light intensity from stars changes with distance.
sciencebuddies.com was recommended to me by my science teacher and it is a very good website that can help you with science projects and even help you decide what you want your project to be!!
eating your butt
Go to sciencebuddies.com it has a lot of good ideas
There are so many interesting ideas a 9th grader can do for a science fair project. One can do a project on whether gum helps you to concentrate, how carnivorous plants digest insects, or the effect of the overpopulation of guppies.
tomatoes and cucumbers
There are plenty of great 8th grade science fair projects. You could play with something like air concepts for example.