Choose a type of plant (tomatoes are a popular choice) and a variable that may affect its growth. For example, place five tomato plants in separate containers, each with a different type of soil, and give the plants equal amounts of water, sunlight and other nutrients. Record the growth (height, width, size of tomatoes) to determine which soil is the most beneficial to the plant. Instead of soil, other possible variables may include type or amount of fertilizer, temperature or light color.
Use the scientific method to determine which brand of a particular product works the "best." Batteries are common products tested in science experiments. Pick a few different brands, use them to power identical objects (flashlights work well) and record which one works the longest. Note in your observations whether the light produced by the various batteries begin to dim or just turn off completely. Other product testing ideas may include popcorn (which brand leaves the fewest unpopped kernels?) or paper towels (which brand is the most absorbent?)
Conducting a science fair project with food teaches not only the scientific method but also important aspects of health and nutrition. Consider a project about storing food safely. A simple project includes keeping one type of perishable food (bread or vegetables, for example) in different settings (dark, light, warm, moist) to determine which factors contribute to or lessen mold growth. For a more interactive approach to a food project, experiment with different baking ingredients to find the recipe for the perfect chocolate chip cookie. Take a basic recipe and add one extra ingredient per batch (eggs, an extra cup of oil, sour cream) and see which results in the chewiest cookie. To ensure accurate results, bake extra batches and ask a group of friends to contribute taste-testing input.
The scientific method need not be limited to measuring attributes of objects. Consider testing personality traits or preferences on a group of human subjects. For example, conduct an experiment to determine whether the appearance of a food affects how we interpret its taste. Gather a panel of taste testers to judge different varieties of the same food while blindfolded. Cookies work well. Ask each person to name his favorite and least favorite choice and record the answers. Repeat the activity again without blindfolds. This time the testers will see that some of the cookies are visually unappealing. (Colored with green food coloring, for instance.) Compare the results from both steps to see if the answers changed once the testers saw the food. You may choose to perform extra experimental stages with additional foods like Kool-Aid or ice cream for more accurate results.