While students are working problems, walk around the room with a handful of green pens. When someone finishes the problems correctly, they get a green star and a green pen. Now they are qualified to coach and star papers for other students, and so on, until everyone is done.
If you think you excel in mathematics and logical thinking, here is a compilation of some tricky riddles which you might just find interesting. The only scenario wherein you are likely to find these riddles easy is, if you already know the answers.