stick a cigarette up each nostril then take photos of them (yes this happened to me)
shave their eyebrows off
put make up on them
cut their hair in a comedy hair cut - never ever do this to a girl or they might actually kill you afterwards!!
cover them in pieces of kitchen roll - someone actually did this to me, i've no idea why it was a bit weird, i think wrapping them in cling film so they're attached to the sofa / bed would be funnier