I won first place overall in the city with my project. Mine was Does caffeine affect you blood pressure? Make sure your project is very neat, and the judges love graphs, use microsoft excel. I used rubber cement to glue the papers to the board, it comes out very neat. Also, make sure you don't have way to many pictures or very bright colors, it's distracting.
You could make a maze for a mouse. See what foods attract the mouse the most. My sis won a science far when she made plenty of graphs, mouse's level of sleepiness, how much water the mouse drank, when the tests were made (day, night) and tel the mouse's age. It may depend on the sense of smell the older it gets. Also test foods like peanut butter, banana, apple juice, yogurt, water (which will have no effect, but the people don't know that, right?), grass, and other food that are not poisonous.