The Secret Service has its own forensics laboratory. We specifically went to the Questioned Documents Section. They learned about all the different spectroscopic techniques that can be used to validate the authenticity of documents. It was great since it gelled quite nicely with what they had been taught in chemistry class.
With the LN2 they did several kinds of experiments, but the one that packed the crowd in was making ice cream. In doing that though I got them to examine the effect of ingredients in final texture, i.e., the different between gelato and ice cream and ice milk. We also talked about the effect of temperature more generally on molecular motions (vibrations, rotations, and translations).