Two things I enjoy doing is to show a few techniques and how they would actually be used as a defense. Then from the front of the class I initiate the offense and have everyone in the class do the defense. Like I slide forward, they slide back. I do a backleg roundhouse, they answer with a front leg round house. It makes them think and keeps their interest. Another is to call off techniques fairly quickly and they do it whenever it is called out. Pushup, hop, kicks, punches, stances, etc. It helps their listening and cardio. If they are having a high energy night, it works of some of the energy so they can concentrate when you move to forms or improving techniques.