The party encounter and defeat a Goblin shaman and his entourage. At the culmination of the fairly straightforward fight, the shaman curses the party with his dying breath. The words of the curse drift away on the late-afternoon breezeâ€¦ but nothing happens.
The excitement begins the following morning, when the party awake to discover that they are no longer themselves. Instead, their souls and bodies have been mixed up. Around the campfire they lie - Shadow the Thief in the body of Rex, the noble Paladin, and vice versa. Juliette, the party’s mage, discovers that she is in the foul-smelling body of Herbert the Barbarian, while Herbert merely stares at his chest for an hour or so.
Actual pairs can be tailored to the needs of your party - perhaps two of your players don’t get on, or you have a munchkin player who is accustomed to wandering into every conflict with their sword swinging. How would they cope if they were trapped in the weak and feeble body of the Halfling alchemist? Remember, the minds don’t change, so the mage in the fighter’s body still knows how to cast spells - but perhaps the fighter’s clumsy hands are too uncoordinated for spellcasting. The thief discovers that the six-foot-six Paladin’s body is entirely unsuited for sneaking and backstabbing - but suddenly, for the first time in his life, he is trusted wherever he goes. Serious issues can also be tackled using this method - perhaps the Paladin believes that, since this is no longer his body, he doesn’t have to stick to his vow of chastity; maybe the surly dwarf in the party is swapped with the gentle cleric, and has to perform a touching funeral ceremony for important local dignitaries.
For fun, try running the adventure as a non-adventure that is, there is no way to remove the curse, no way to change back into their own bodies, and nothing to be done. Perhaps it’ll run out, perhaps it won’t, there’s no way to know. How do the characters react? How do they behave? And how do they work together as a team when the Goblin Shaman’s eldest son comes with a raiding party to seek revenge?
Convince your players that the change is permanent, then reverse it again the following morning, after the Shaman’s son has been defeated, or whenever you think it would be funny. It’ll leave you with stronger bonds within the party, and hopefully, better roleplayers.