At regular intervals, a god takes control of your world's weather. Choose the gods at random each turn of the interval and begin applying their effects subtly. Their power will depend on the strength of those who worship them.
For civilised people, worship of a god actually wanes during that god's month in an effort to reduce their potential destructive effect.
A kingdom where, if there are multiple princes (princesses can't rule unless they have no siblings), the princes fight to the death in the arena to decide who gets to rule. A simple way to make sure your brother cannot make a bid for the thrown, and keeps the royal blood in one line.
A country where noone can be executed whilst telling a story provided it's reasonably interesting.
A world where the minds of the rich and the dying are transferred into the bodies of convicted felons.
If you possess someone, you have an hour to get back to your own body or your thoughts will be overwritten by that of the orginal person
Magic is like alcohol, the more that is used, the more it causes a hangover later on and the less judgement one has when using it. If one waits a while after casting a spell, things "detoxify." A cantrip or two is like a sip of weak beer, whilst a large creation spell is like a bottle of vodka. Cast something too big and you can die from magic intoxication.
For the people of Kuboloth, hell is not in the depths of the earth but in the cold of the windy air;heaven is in the warmth of the earth, lit by glowing jewels.
It is possible to buy magical abilities. The process is very dangerous though, involving chemicals, rituals and surgery possibly leaving the aspiring mage insane, retarded or scarred for life.
Magic is impossible except for a handful of days in the year, when those with the knowledge become hugely powerful.
What to export or import....
Fabric: wool, linnen, silk...
Wood: raw-material, furniture
Metals: iron, gold, silver, copper...
Wine, beer, mead, spirits...
Animals: horses, sheep, swine, cows...
Coloring-powder (for fabric, ink...)
Raw-material to make fabric: unprocessed wool, linnen, silk...
Glass: Windows, figurines, glasses, raw-material...
Tea (not sure about my spelling here, but I mean the hot drink Englishmen drink instead of coffee!)
The more you dominate your life to a god or goddess, the more you gain magical powers from him or her, but the big stuff does not come easily. Most worshippers have only the weakest of spells.
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.
In ancient China, women bind their feet for cosmetic reasons, resulting in them not able to walk without help and walking in a shuffling gait.
Possibly this can be modified such that people do it for less stupid reasons.
Disclaimer: This is NOT a racist remark since I'm Chinese myself and hence entitled to freely judge Chinese customs. NOT sexist either since I'm female as well.
Magic is addictive, small spells are like cannabis in addictiveness, major ones are like cocaine or herion. Many countrys ban even cantrips as a result.
A planet (or city, valley ect) where those who enter witness protection can escape old age and death itself, so they can live for centuries without aging-provided they maintain their cover perfectly. If they are outed by anyone or ever utter their true name, they age the number of years they have not aged since the day they entered witness protection (which might mean that they age centuries in seconds and crumble into dust and bone.)
Elven prison sentences even for small offenses seem very long to humans, but this is not because their rulers are draconian, but because elves live so long that a six year sentence, for example, is like a six week one in human terms.Humans in Elven countries are well advised to behave themselves
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 land where virgins are immune to magic (although they cannot cast it.)
In the dry steppelands, one of their most valuable exports is the dried sap of the Larthorn tree. These ugly plants are covered with vicious thorns, but the locals harvest the golden droplets that ooze from their bark each Autumn. This sap, once dried, is valued for its medicinal properties and as a spice. Since little gold or silver is found in the hinterland, the dried droplets of sap are often used as currency by the locals.
A terrible teleportation incident makes inter-demihuman chimaera of all the PCs. Quoth Master Adams: 'I teleported home one night / with Ron and Sid and Meg / Ron stole Meg's heart away / and I got Sidney's leg.'