If you have an old LCD display you can remove the polarized and anti-glare films from the inside of the monitor's glass surface and reassemble it; this will make the screen look bright white to the naked eye. To see the actual display you just need to use a bit of paint thinner to remove the anti-glare film from the polarization film and cut that to match the frames of an old pair of glasses. This produces a monitor that can only be seen when you're wearing the polarization filter glasses.
Instructables user dimovi provides step-by-step instructions and photos that describe how to remove the polarization filter from the display and cut that filter to make lenses for disposable 3D glasses or old sunglasses. I'm curious to see if regular polarized sunglasses would also be able to view the hacked display.
Privacy monitor made from an old LCD Monitor | Instructables via Hack-a-Day