You’re very, very bored, and you have lots of boxes of useless business cards.
So you build a Menger sponge out of them. The instructions are online — you fold each card into a module, then interlock the modules into cubes, then interlock the cubes into larger cubes, fractally, until you’ve got something that looks like a three-dimensional fractal cheese grater made entirely out of folded business cards.
I’ve only managed one “level one” so far. People at my office have stopped asking about things like this, after I hung the geodesic dome I made out of soda straws from the wall.
The dome was better. But the Menger sponge uses up more business cards.