According to Science.com, exploring how long it takes for drinks to stain a person's teeth is a good science project idea. Testing the relationship between a person's eye color and sight is another good idea for a seventh grade science project.
To determine how long it takes for drinks to stain a person's teeth, pour coffee, tea and cola into three large containers. Place at least one egg shell in each container. Take it out every day to observe the changes. Take photos, and record the changes. Compare the results, and make conclusions.
Egg shells have structures similar to that of human teeth, so the drinks have the same effect on them.
To test the relationship between a person's eye color and sight, gather about 15 people with each eye color (blue, green and brown). Make sure that nobody is wearing glasses or contact lenses and that the participants are about the same age. Tape several sheets of colored paper to a wall and dim the lights in the room. Blindfold the participants, bring them into the room, and have them stand on the side of the room opposite the colored paper. Take the blindfold off, ask them to name the colors of the papers immediately and then again in two minutes. Record the results, and calculate the average percentage of the colors named correctly for each eye color immediately and after two minutes. Note any difference between the groups, and make conclusions.