Scene: Linda (Kate) & Johnny (Cary) doing their little acrobatic routine in "Holiday."
(I believe this is a different scene than the "flip-flop" scene already claimed, yes?)
Another Scene: Susan (Kate), David (Cary), and George (Asta) singing in...er..harmony "I can't give you anything but love, Baby" to the leopard on the roof in "Bringing Up Baby."
I hope I did that correctly!! ;)