Each year there's another batch of science fair pleas in my mailbox. "Help! My daughter (or son) needs a science fair project. Can you suggest one?" Each year, my answer is always the same: no. I make a point of not doing my own kids' schoolwork -- so don't expect me to do your children's homework. But I do know where your kids can go for ideas that will get their own creative juices flowing. And that I'm very willing to share.
From astronomy to zoology, All Science Fair Projects is a searchable database of 500 science fair ideas for all levels (elementary, middle and high school.) You can search by keyword (such as "bacteria" or "sun spots.") Or browse by topic (biology, chemistry, physics, earth science and engineering.) Each individual project page then links to an sample project elsewhere on the Web. There is also a good resource section that includes links to a few of the large state science fair sites, such as California and Chicago.
Science Buddies is a non-profit organization encouraging students to "improve their science skills" and "consider additional study or careers in science." It's also my pick-of-the-day site because of the Topic Selection Wizard, and the general excellence of all the guidance provided. Choosing a topic is often the most agonizing part of starting a science fair project. Use the wizard to guide you toward a topic that interests you. Be warned, however, you'll have to answer a lot of questions to work your way through the wizard. Other great clicks are the list of bad science fair topics in the "How To" section, and the "Ask an Expert" online forum.
Although there's lots of fun science stuff to peruse here, you'll find the meat of the matter in the Soup to Nuts Handbook written by Janice Van Cleve, author of more than forty books on science and science fairs. "A science project is like a mystery in which you are the detective searching for answers. Science projects let you practice and exhibit your detective skills. You not only get to select which mystery to solve, but you can creatively design methods for uncovering clues that will lead to the final revelation of who, what, when, where, how, and why."
"Students are advised that getting the right answer is NOT the purpose of a science fair project. It is the intent of a science fair project that you go through the process of asking questions and performing experiments in an attempt to find answers. Making the attempt without answering the question still satisfies the intent of your discovering knowledge on your own." From the California State Science Fair, comes this useful discussion of quality in a science fair project.