May I recommend building a centrifuge? When I was in middle school, Scientific American had a cool article about extracting DNA In the kitchen. It was an abysmal failure, but attempting to build a centrifuge to separate the samples was immensely fun. Coolest, most thrilling project I tried in school (messy, too).

Note: blender wiring can catch fire if they are loaded for extended periods of time (don't underestimate the inertia), but old blenders work fine for at least a few minutes. Lego Technic works great, but only with really solid cross bracing. I recommend building a barricade around it (World Book volumes work nicely).

Edit: I should note that it did work. The rig spun two 10mL test tubes on swing arms at the low(est?) speed setting (which is pretty fast at a 4 inch radius). It seemed to work, but I never did figure out if the scum on top was DNA.