Anti-bacterial experiments using standard products or solutions provide one good science fair topic. For example, you can test the effectiveness of anti-bacterial soap, toothpaste or ointment. Try using different brands of each product or different chemical compositions to determine which one has the highest value. Compare that to the advertised statements. For example, test the claims that a soap can eliminate 99 percent of bacteria on your hands.
Advanced students can study Polymerase Chain Reactions (PCRs), which generate millions of copies of a single strand of DNA. It is a new reaction, only first conducted as recently as 1983. Students can design experiments that determine the optimal conditions for these reactions. For example, you can conduct these experiments under different temperatures, concentrations or air pressures.
Another experiment that can be conducted is mutation detection. Inevitably, in any organism with billions or trillions of molecules there will be many mutations. These can occur in all of the different parts of a cell. A student can experiment to find new ways of detecting, counting or reducing mutations. Try different conditions or compound applications based on your knowledge of the subject to complete a project.
Be sure to read and carefully follow science fair guidelines. Many fairs actually place just as much importance on the creativity and process of the experiment as on the actual results. Most fairs will not expect you to produce groundbreaking results on a shoestring budget in the span of a few months. Instead, they are probably looking for your thought process, original idea and ability to document results properly. Focus on these parts of the experiment just as much as trying to produce a breakthrough result.