Beside the road stands a large oak tree that the PCs must pass by as they approach a small country village about a mile or so distant.
Standing under the tree is a young woman, maybe late teens or early twenties. Her hands are tied behind her back and her ankles are tied together. There is also a noose around her neck, the other end of which is tied to the lowest branch of the tree, but with just enough slack that she may stand without being strangled.
As the PCs approach, the girl will beg them to untie her.
The girl is a criminal and this is the local form of execution. The (somewhat gruesome) idea is that she has considerable time to contemplate her own end before she becomes too exhausted to stand up straight, at which point she will collapse into the noose and die of strangulation.
The nature of her crime is left entirely up the GM (anything from stealing a loaf of bread to mass murder, depending on just how draconian he wants the local justice system to be).
The girl is a sacrifice to a local demon or demigod and if she is not here at the appointed time (i.e. the PCs rescue her) there will be terrible consequences for the nearby village.
If she is to be sacrificed to a demon then this is price for appeasing it. If the PCs rescue her it goes on the rampage and attacks the village. The PCs now have a demonic monster to fight.
Alternately she may be intended for a minor deity, in which case her life energy is what the spirit needs to ensure that the villagers’ crops will grow.
Might also tie in (excuse the pun) with option 1 (we need a sacrifice anyway so we might as well use a lawbreaker).
The girl is part of an outlaw gang who prey upon travellers along this road (her bonds are quite loose and she is perfectly capable of removing them at any time). In this case as soon as the PCs come to her aid the rest of the gang (several large fighter types) jump out of hiding and attack.