Favorite liquid nitrogen trick: Fill a heavy dishwashing glove with red Jell-O, then put on a similar pair of gloves and do a few standard liquid nitrogen tricks (dipping a rose, etc.). When people get used to the idea, pretend to drop something in the vat, cuss and reach in to get it. The audience will scream. You, meanwhile, drop the Jell-O-filled glove out of your sleeve and dip that in the fluid. Pull it out, steaming, and say, "Wow, that's cold!" Then slap it on the tabletop, sending frozen red chunks spinning all over the table.

This gets a great response.