This has just occurred to me...
What's the best way to demonstrate the difference between a Retina-screen iPad and a non-Retina-screen iPad?
Open an iPhone-only app on the iPad, and press the "2x" button.
The difference will be so brutal it will make your eyes pop up.
On a non-Retina-screen iPad, such an iPhone app will be grainy like hell. It will literally hurt your eyes to look at it. But on a Retina-screen iPad, the same iPhone app will show no graininess. (Though it still won't be optimised for the larger screen, of course, so its use on an iPad can't be recommended.)