i did a pretty cool one one year. it was all about stars and constellations. i mapped out a picture of the north sky as seen this time of year and took a giant sheet of wood (a little larger than the size of a bristol board) and drilled holes in it and poked tiny christmas lights through it to represent the stars. i drilled the holes where certain stars from different constellations would be located and put a bunch of black construction paper over the front of the sheet of wood to make it look like a night sky. looked pretty cool. definitely attracted attention.
and then in the presentation i described stars, their life and death, phases they go through, some of the constellations and what they mean, where they originate, all that kind of stuff. fun times.