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.