This is an activity we typically do when learning about algorithms at the beginning of the year, but it's fun and more advanced students will hopefully do a better job at it.
Part One: One volunteer comes to the front of the class. She sees a simple drawing on a piece of paper of several shapes (triangle, square, circle in one example) next to each other with slight spaces in between them. Everyone else in class has merely a blank sheet. Her task is to get them to reproduce what only she sees. The catch is that everyone else in class has to think like a computer and make no assumptions. Saying "draw a circle" or "make a line" won't accomplish the task. (Side note: this could be a great way to differentiate imperative and declarative languages, but that's a separate topic.) Computers need to be told to put pen to paper, lift pen, etc.
Part Two: A different volunteer comes to the front of the room. The task is reversed. Everyone else has a drawing in front of them, and they need to communicate with the volunteer to get her to draw it properly on the whiteboard in the front of the room. The same rules apply. Classic "bugs" that come up here are not instructing the student to take the cap off the pen and not having her lift her pen correctly to complete the drawing, which is typically a stick figure with a word bubble message.
Both activities are engaging, and do give a number of opportunities to step in and talk about assumptions we as humans make and about abstractions that we may take for granted, like "draw a shape."
I got this idea from my time taking CS50 and being trained to teach CS50 AP. You can see this activity demonstrated at Harvard here.
Also, there's always the classic "Peanut Butter and Jelly Sandwich" activity although this one is hard to do at the last minute. Watch the CS50 at Yale demo here.