Everything Kol Korran said, especially the changing into monsters thing. Especially if there are races like elves and dwarves and half-orcs and halflings. Imagine if suddenly the elf was a Dwarf, or the halfling a hulking monstrosity. In fact it would be really funny just to do a nice old fashioned mind swap. All the same characters but with each other's bodies.
You could also always turn the paladin and/or good cleric into a demon. That would be funny.