Soothingly restful easy listening music: not music to sing to or dance to, but music that just sits in the background, unobtrusive but comfortingly and tastefully there.
Your selection, like your choice of wallpaper, will of course be determined by your own personal style. I go for plain walls and electronic musical drifts of sound, e.g. Amon Tobin. My father goes for stripes, and jazz.