Write this down on three pieces of paper and then leave the papers throughout the classroom:
Why is there a footprint on the ceiling?
Pass it on
That is guaranteed to cause all sorts of mischief as people pass the note around and try and locate the foot print.
Alternatively if your instructor is late you could do this:
Walk to the front of the classroom and write on the chalkboard.
"I will be late today. Please write an in-class 500 word essay on chapter four. I will collect it at the end of class.
Announce that Professor Hackleshmackle is going to be late and has asked you to lead a discussion.
Take out your textbook and start randomly calling on students. Ask the students you call upon to comment on a particular section or chapter and then tell them that they are wrong and that if they had attended the last study session they would know better.
Be prepared to run.