Who knew the photo-graffiti mode of the DSi could turn into a poor man's KORG DS-10? Dover, from the site/webcomic Monday Night Crew, found a genius way to compose piano tunes. Watch.
I looked on the site for the musical scale picture he created, but couldn't find one. Still, it doesn't look that hard to reproduce. Of course, this depends on an ability to read music or recognise notes, but if you've got that already, then impressing your friends is the easy part.
+1 for usage of the Millennium Falcon pic. And the Zelda theme, instead of Super Mario Bros.