The best graded science projects for the seventh grade usually revolve around chemistry. Experiments dealing with liquids and gas tend to be the highest rated.
One of the most popular and best graded experiments is to create a self-inflating balloon. Fill a bottle or test tube halfway with vinegar. Using a funnel, pour a teaspoon of baking soda into a stretched balloon. Place the balloon over the opening of the container and seal it shut. Shake the balloon to make sure all of the baking soda falls to the bottom.
The reaction of the vinegar and baking soda should cause the balloon to inflate.