A fun adventure that I have run is having the player group act as bodyguards for a noble or politician. Sometimes I have the noble be competent and basically nice to the group, and other times he is an idiot who abuses them at every opportunity. Either way, they have to keep him alive.
Assassins I have used included another well-rounded adventuring party, archers using poison, and someone loosing an owlbear into a crowded market where the noble was shopping (despite the players protests about him doing this).
It can be a very interesting dynamic. Players have to risk themselves, sometimes even jumping in front of an attack, to protect someone else. They don't just line up and kill everything. They have to look out for someone else, who often has no protection (and once only had three hit points and no armor).
It definitely caused some strong reactions from my friends. A couple of them loved it. The sit-there-and-roll-dice-but-rarely-talk dwarf player hated it.
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