This site is full of science projects with full instructions on how to complete them. It’s geared for all levels of students and you can use their handy dandy search engine to search by topic. Or you can click the big orange circle that says “Browse” and view a list of all topics.
By clicking the Browse circle you are taken to the listing of all the topics. Here you can even browse by the type of science you want to do a project for: Biology, Chemistry, Physics, Earth Sciences, and Engineering.
I loved the “Food Science” category under Chemistry. My favorite under this category was the Popcorn experiment. The point of the experiment is to find out if different brands of popcorn have varying amounts of un-popped popcorn kernels.
You can tell which level the project is by looking at the top of description where it says difficulty. There is also a really wonderful encyclopedia for help with terms and concepts.