I remember singing a carol to the melody of Personant Hodie at a Roman Catholic Mass some years ago; can't remember what it was though... Let us sing, sing, sing;let us sing, sing sing... Something like that maybe? I remember thinking how nice it was to sing the melody in context rather than as an exercise in early music, which inspired me to try the rounds from the Llibre Vermell with a class of boisterous 11-year-olds - Splendens Ceptigera & Laudemus Virgenem - which I've also used in folk-club singarounds. Great icebreakers...