Sounds like a fun assignment. As a physics graduate, I feel obliged to make some suggestions:
- You could look at planets for gravitation and theories of planetary formation. Mercury is particularly interesting, as its orbit doesn't quite agree with Newtonian gravity and provides empirical support for general relativity (google "advance of perihelion").
- Galaxies have all kinds of physics going on - gravity holding them together, dark matter explaining rotation curves, redshifts giving us the Hubble law, black holes in the center of some;
- Nebulae could involve a discussion of emission and absorption spectra, including classical electrodynamics in dispersive media;
- The sun - plasma physics, gravity, electromagnetism, nuclear physics.