This site provides a number of different ideas for science fairs projects. It even includes a topic wizard questionnaire that helps your students narrow down their interests. (Select the menu tab "help me find a project.")
Projects have been categorized under 7 major headings with a number of subtopics. For example, under the major heading "physical science", there are subtopics such as cooking and food science, chemistry, and sports science. For each project, there are a number of tabs such as summary, background, materials, and procedure. This site provides enough information for students to design their own exit project or science fair project.