On the first day of Christmas my kitty gave to me
A poop in the box

On the second day of Christmas my kitty gave to me
Two lumps of urine
And a poop in the box

On the third day of Christmas my kitty gave to me
Three slimy hairballs
Two lumps of urine
And a poop in the box

Ok, that’s all I’ve got.

Hmm… just realized that I based the last line of each verse (“And a poop…”) on the somewhat non-standard metre of the Bob and Doug McKenzie version (“And a beer… in a tree.”)