Here are some ideas on questions to ask. These same questions can be about different things in each of the categories in the Science Fair. The best projects come from investigations of questions the students are interested in. This can range from questions in "real" science to sports, clothes, food, pets, TV, games, friends, etc. Remember that a good Science Fair project has an experiment at it's heart.
Students should avoid doing demonstrations and collections. Judges are being asked to give only the top scores to projects with experiments. Still want more concrete ideas? Check out some of the web sites in the Other Neat Places to Visit page.