“Santa Claus Got Stuck In My Chimney,” which Ella Fitzgerald manages to sing so innocently that one might almost believe that she was unaware of the innuendo.
“O Little Town of Bethlehem” and “It Came Upon a Midnight Clear” are both in ballad meter, and the words of all ballad meter songs (including House of the Rising Sun, the Gilligan’s Island theme, Yellow Rose of Texas, and a long list of others) can be sung to each other’s tunes.
They can also be sung to the tune of “Joy to the World” (but not the reverse) and they, along with “Joy to the World,” can be sung to the tune of “Stairway to Heaven” (ditto).
ETA: I’ve always wanted to see a group of carolers sing the words of “O Little Town of Bethlehem” and “It Came Upon a Midnight Clear” to each other’s tunes, to an unsuspecting audience, just to see the reactions.
Doctor Demento played a version just like that every Christmas. I’m pretty sure it was originally made just by slowing the Chipmunk parts down, long before programs like Audacity became available. For several years I’ve been meaning to use Audacity to only lower the pitch for the Chipmunk parts, but I never got around to it.
“Because I Could Not Stop For Death” works amazingly well set to the music from HotRS.
Some really weird stuff on here: Tammy Faye Bakker, MECO (as “Star Wars Christmas”), Esquivel, the previously mentioned Martin Mull, etc.