This activity exposes the idea that interesting things can be achieved using a simple instruction set and components, giving an idea of the underlying architecture of the computer. Variation: introduce a bug in the code, and see how long it takes the class to realise that things aren't right Variation: write some code to generate an interesting output (such as the Fibonacci series) and see if the "computer" can figure out what the program does.
Misha Leder, a Software Engineer at Google has an activity called Simulating a Computer where kids play main computer components - CPU, mouse, IO controller. Have them simulate calculator application by moving a mouse, synchronizing it with the mouse pointer on the screen and sending request to the processor.
Computer Science & Engineering for K-12 (cse4k12.org) has the following activities related to how computers and internet works below:
Kaleberg has a page on Cardiac ( Wikipedia: Cardiac). Cardiac was a little cardboard handout designed to help explain how computers worked. Since it was made of cardboard, you entered decimal numbers into memory by writing them in the appropriate slots. You could use pencil for RAM and pen for ROM. Then, you followed the instructions on the card which told you how to interpret the computer program. Download and read more about Cardiac at Software Emulator for the Cardboard Illustrative Aid to Computation .