If I'm working with elementary school kids, I list all the work the class needs to get done that day on the side of the board. I tell the students we just might have some free time near the end of the day IF we cover all the material the teacher has left for us. After that it becomes a game-- us against the list. As we complete each part of the lesson plan-- say Math-- I erase it off the board. We almost always end up with a little chunk of time to spare because the students were motivated to stay on task. We usually play "Heads Up Seven Up"; that's always a big hit. If the class doesn't know how to play, I teach them! 
