*LOL* you could really screw with your PC's and put them in a "Groundhog Day" effect. I also saw this on a Xena episode, and it might be kinda funny.
The PC's have just finished destroying the graveyard, and all is in goodness. Have them return to the town for a well-deserved night of rest. When they wake the next morning, everytihng is as it was... yesterday.
This will continue to happen everyday until the true source of the undead menace is found.
This requires you to think in advance to find out the chain of events that leads back to the real source. You could save time by using any or all of the examples potsed above --
a) The undead in the graveyard are dead. If the undead are not destroyed, they destroy the village in turn. Sleep.
b) They learn that the undead are not from the graveyard, but from a neighboring village that is being stirred up by Paladin Bob of the Shiny Order (TM, Patent Pending). Get Bob to stop killing undead by finding the Unholy Artifact he's looking for. If the evil doohickey is not found or Bob doesn't stop killing undead, go to a). Found, destroyed, yay. Sleep.
c) The Unholy Artifact is a maguffin. It was placed there by the King who is testing a new weapon that creates undead; the item is actually akin to a homing beacon/laser designator that helps the king pinpoint where to use the weapon. Find the source of the attack, find the evil weapon, destroy weapon, yay, sleep. If the weapon is not destroyed, return to a).
d) The weapon doesn't actually work. The TRUE power comes from a Lich, come to the King in disguise. The Lich has charmed the King and has the King convincingly create the "weapon", which is in fact a minor magic item that signals the Lich that the King is about to use the weapon. When the king fires, the Lich uses its own magic to create the undead. When the citizens learn of the terrble weapon, they send for outside assistance. PC's come, as well as an assassin. If the assassin is not discovered, he/she slays the king. Here the PC's might have to split up and search for the assassin; half goes to break the weapon, half finds the assassin. If not...
e) If the assassin is NOT stopped, the King is killed; The Lich then takes the form of the next in line for the throne, ascends, becomes ruler. If the assassin IS killed, weapon is still fired, undead created. If both goals are not met, go to a).
To end the repetition, pc's must learn tha the Lich in disguise is the true master of it all and find a way to defeat him. Have an 18 hr time limit in effect -- if the PC's battle on beyond this time, they feel a compulsion (like suggestion) to fall asleep. When they next awaken... back to square one.
I don't know how this would work in play, whether the PC's would get all pissed, but they would have the advantage of learning more as each day passed. Thoughts?