I helped out with a middle school astronomy event on Friday evening - it would have been a star party except for the solid clouds and rain. So it was all indoors, and my activity was crater making rather than my usual "exploring space with a computer" show. Crater making involves a bin with a thick layer of flour topped with a thin layer of cocoa powder (for the contrast and the nice smell), plus a few rocks of various sizes - impactors. Drop rocks into the planetary surface and make impact craters.
This may sound a bit silly, and it's not completely realistic since the projectiles are moving pretty slow and don't vaporize or explode (safety first!). But you can see the effects of the mass and velocity in terms of the crater size and depth as well as its shape and the ejecta pattern. Eighth graders are surprisingly involved with this, especially if they get to drop the rocks. I show some photos of Moon craters, and some of them look remarkably like our flour/cocoa ejecta patterns. I ask a lot of questions and try to get the kids to think about the possible importance of impact craters and the objects that made them, as well as the fact that the Moon seems to have more craters than the Earth. Why would that be? Where did our craters go? (Most say in the ocean - pretty good guess on an area basis.) It's more fun and thought provoking than I first thought it would be (I've done it about 3 or 4 times now, so my setup and delivery are getting better).
I looked around for some impact crater information on the web for backup and preparation, and there's quite a bit you quickly find with Google. One that I especially like is this University of Arizona site, "Terrestrial Impact Craters and their Environmental Effects." The material is interesting and reasonably detailed without being too technical. The animated GIF above is linked from this site - very cool (click on the image for a bigger version - you may have to shift-reload this page to force the animation to play or replay). Explanation and connection to dinosaurs can be found here.