I believe it was Garrison Keillor I heard say that, as an English major, he learned that nearly all of Emily Dickinson's poems could be sung to "The Yellow Rose of Texas," as in: "Because I could not stop for Death, he kindly stopped for me ... ." Try some others, too.

Also, I sometimes switch tunes and lyrics between "This Land is Your Land" and "You Are My Sunshine".

The Easter hymn, "Christ The Lord is Risen Today" and the Christmas carol "Hark The Herald Angels Sing" can be switched (with a little tweaking, such as adding the "Alleluja" part to "Herald Angels.")