The Christmas hymn, O Come O Come Emmanuel, is a lovely piece using minor keys. I've always wanted to do an arrangement of it for a solo performance.
This piece is fairly simple and would work on piano or organ. The vocal range would be second soprano to first alto range, or suitable for baritone or tenor by moving the melody down an octave.

Lyrics:
O come, O come, Emmanuel, and ransom captive Israel
That mourns in lonely exile here until the son of God appear.

Chorus:
Rejoice! Rejoice! Emmanuel shall come to thee, O Israel.

O come, thou rod of Jesse, free thine own from Satan's tyranny.
From depths of hell they people save, and give them vict'ry o'er the grave.

O come, O Dayspring, come and cheer our spirits by thine advent here,
And drive away the shades of night, and pierce the clouds and bring us light.