How about the enormity of space, the actual emptiness of most of the solar system or how about the various size of objects such as stars from small neutron stars tens of miles across to VY Canis Majoris, over 1 Billion times the size of our Sun. Good luck.
P.S. Since most people are visual in nature, I recommend going to YouTube and looking for video's on different astronomy subjects!
I did my 12th grade research project on the planet Jupiter. There's plenty of information about it, and you'll learn a lot about how it aided in the shift to a heliocentric view of our 'universe,' how the speed of light was initially estimated, the existence of trojan asteroids, and many other topics. You can learn about its thin ring system, its many satellites, its effect on comets and asteroids, and of course, the Great Red Spot. Missions like Voyager, Galileo, and Cassini also increased our knowledge of this planet and its system.
Other interesting topics, with a lot of avaliable information, that I doubt many people will chose:
Cataclysmic Variables
Edwin Hubble and standard candles, along with the discovery of redshift... and the impact those discoveries had on our view of the universe.