Bridges. Each winter, 5th graders measure two carrots--one large, one small--and make predictions about what will happen to them if left out for a month. My kiddos LOVE this experiment. Our only regret is that we didn't take regular photos of the ensuing deterioration; it's really quite remarkable to watch the "shrinkage!"
Maybe you'll want to try a similar experiment. Post your photos...my students would love another chance to get grossed out! :)