If you feel that the Solar System is too simple or just plain boring to make your science fair model, how about trying one on the life cycle of a star. You could chronicle the various stages in the birth and life of a star and make related models for them as part of your science project.

You could talk about the different processes that are in action as the star goes from protostar to a white dwarf.  In the display you could use the information about the different types of stars recorded and give images for them. You could even explain the inner mechanisms of the star by giving the chemical processes that take place within the star.

Explain how the interstellar dust becomes a twinkling star. You could even do an in-depth analysis of our own Star, the Sun.  Maybe elevate fears of the sun dying on us as well. You can talk about why stars appear to twinkle when seen from the earth, but do not twinkle when you are out in space. The stars can give you enough material to make a number of science projects many times over.

It is up to your to choose what you wish to base your project on. There is great scope for Topic Research in this field and you can get help with organizing your research if you follow the link. Go from gazing at the stars to explaining how they work.