Let's create something similar to this and this. You can use a textarea or pre to preserve the formatting (html will otherwise strip leading and trailing spaces). Every time the user presses a key, you can add another character to the displayed text. If you use a textarea, use event.preventDefault() to stop the original character from being typed.
Bonus points for "windows" (div with position:absolute) randomly opening and closing with their own text. They could have their own text. You could even use a canvas to draw some patterns or charts.