"If the Eyes are the windows to the soul, then his soul was burning in Hell."
It was common belief in medieval times that goods and property could only be affected by the Fair Folk, the faeries, if it was acquired unlawfully or unfairly.
The gold hoops and rings that sailors wear are to throw into the sea if there is a storm. These are bribes to the sea gods to let them live through the storm.
Once a year, the king steps down from his throne, and lives as a peasant for one day, the next day is the anniversary of his coronation, and it is celebrated by a reenactment of the ceremony. This is doubly beneficial, firstly, because the people are able to have the grand parade that always precedes the ceremony. Secondly, because the heir to the throne is given a day on the job every year. Although, apparently, the thought has not yet come to one of the kingdoms enemies, the assassination of the king during the renewal coronation would be a moral, as well as a political blow to the kingdom.
A world where, instead of aging in years, the people age in knowledge. So, instead of dying once a general age was reached, they die once they achieve a certain amount of knowledge. A man could live forever if he never learned anything, or could die after reading a few choice books. Of course, the knowledge of the land would be regulated, like medical drugs.
What if a invisible person could see other invisible creatures? All you would need to do to check for nasties hiding around is turn invisible.
What if you send the players in a dungeon that was already raided? Maybe you could have traps that are already set off, or the corpses of monsters etc.
A place where an architect has built a temple/shrine to an evil god inside the walls of another building, a castle or another place of importance.
An adventuress, Semma, has returned to her home village, but something has gone badly wrong: it's the furtive glances, the conversations that stop when she approaches, the childhood friends who now have no time for her. She returns to the town or city and enlists your party to help her find out what's going on.
The party must find out what is wrong: something (perhaps a Cthulhoid monster, perhaps a gang of vampires, perhaps just a bunch of bandits) is extorting obedience from the villagers by threatening their loved ones with at least death. Once the source is discovered, Semma and the party and a few brave yeomen (and women) must deal with the threat by finding its base and defeating it, and then deal with the remnant corrupted villagers who willingly served the Evil Force. These may prove to be the most difficult foe...