This is a short activity I developed to help my students interpret Material Safety Data Sheets (MSDS). Students rank unknown compounds from least safe to most safe, and the compounds' identities are revealed. The class discusses other factors to consider in evaluating the safety of an experiment. The "MSDS activity" word document contains a more full description.
I've only done this in a class of six, but I think it could work for larger classes with some adjustments. The activity gets everyone talking to each other and standing up, which is always a plus.