I went back to the toothpickase lab with AP Biology. It's super easy to find in a quick websearch. The first day I didn't have bowls - that was a mistake. I found these little bowls at Target for less than a dollar each, it made such a big difference. The kids start with just toothpicks and then add in paperclips to distract them as inhibitors. I went ahead and bought 8 big boxes of toothpicks so that I would make sure I had enough for at least next year. (I am so tired of going to the store for lab supplies.) I had one group that was really serious about not peeking during the trials.
I had one group that night have been a little overly organized (I loved it).
I avoided the cold water segment because there was an incident one year with students trying to see who could keep their hands in the cold water the longest (not at my school ... but still ... keep an eye on that).