Horribly corny, schmaltzy, embarrassing songs. I've made it a habit to check whether my earphones are really plugged in whenever I listen to music outside since there's no way in hell I want people to find out what I sometimes listen to if I can help it. (Never mind that the music automatically stops when the earphones are unplugged.)
I also often take them out of my ears to hear how loud the music is from the outside so I know whether someone sitting next to me on the bus would hear anything. Yeah, I'm that paranoid.)