A land is reigned by a circle of powerful men who control every aspect of the citizen's life. This cabal changes members often. In reality, the spectres of a small necromantic covent possess and control the people in power. Since they simply possess the bodies, they can leave when old age overcomes their shells and possess a new up-and-coming noble.
A rich southern landowner once imported hundreds of thousands of spiders and distributed them along his long, tree-lined road. After a couple of days, when the spiders had make an incredible number of webs amongst the rows of trees, he spread fine gold and silver dust on them to create a spectacular shimmering view. (true story) It was for his daughter's wedding day.
There is a land where every person wears spectacles. Those without eyeglasses are considered to be the lowest stratum of society, so adventurers without glasses are treated like outcastes. It certainly would explain those heroes wearing sunglasses for no apparent reason...
The player characters, experienced and somewhat well known, hear rumors and travelers' tales about a distant area being overrun by dragons (or other terrifying monstrosities). The locals have sent them a message, begging for heroic aid.
When they investigate, they discover that nothing of the sort is going on. It turns out that a group of thieves wanted them out of the way so that they could rob them (or someone who would normally receive their protection).
The PCs encounter an elaborate trap room, designed to fill with water, drowning those inside. Clever and resourceful, they disable the trap and move on.
Only later do they discover that this room is part of the elaborate cooling system that prevents beasts of elemental fire from overrunning the complex. While the fiery beasts run rampant, they begin setting fires, which causes the complex to become increasingly hot and smoke-filled. Additionally, the PCs now have to fight their way out past aggressive fire creatures...
An alchemist enchanted his equipment to produce even more powerful alchemical compounds, potions and the like. Unfortunately, it is not well-known that he had different sets of equipment for different processes: If the wrong set of gear is used, the result will invariably fail to function. It's not a cursed item, just something that's hard to figure out.
In a city where the justice system features judicial dueling, plaintiffs and defendants are permitted to request a champion to take their place in the duel: Someone chosen by lot from among the foreigners in the city. When anyone first arrives, they are given an enchanted ceramic pendant that marks them as a candidate for "court duty".
Wealthy folk entering the city are often escorted by burly guards, paid to carry pendants on their behalf: They elude court duty in that way.
Adventurers may seek work as a rich man's proxy or may find themselves magically summoned to serve as a champion.
Certain trees have sap that is filled with magical potential. Their sap is harvested and distilled down to produce various types of potions, much like maple syrup is collected in some areas. The magical effects of these syrupy potions are often fairly trivial; perhaps they simply cause the imbiber to grow hair more quickly, summon butterflies, or walk a few inches above the floor. People enjoy the syrup as a flavoring, instead of imbibing it for its magical powers.
A powerful wizard that the PCs encounter is completely silent. He does not speak and cannot be heard as he moves. Even his spells are silent, but his magic is just as potent.
He has a tendency to summon invisible chairs, place foes in invisible boxes of force, and other "mime tricks".
A desolate region is almost entirely without normal vegetation. Local plants are able to unroot themselves and crawl along the ground in search of water and fertile soil. The inhabitants fence their crops in to keep them from wandering off and put heavy stone thresholds in the doorways of their huts to keep wayward plants out.
The plants sense by chemical cues, lacking sight or hearing, and tend to avoid herbivores or anything that smells of "dead plants". Characters with horses are likely to be unwelcome among the locals.
Giant mushrooms the size of trees. The stems and parts of the caps can be used in construction, and only the gills are soft enough to eat. An entire fungi-forest can grow in a matter of days, provided the correct conditions. Most fungus trees last about a month before collapsing, but not before a new generation sprouts to provide shade for the next spores.
Near a major city, a mirror image of the place has appeared. This strange double is infested with hostile insectoid humanoids that appear to match the city folk in numbers and armaments. The humans’ diviners have determined that the secret of the city’s appearance can be found within the double’s college of magic, so heroes are needed to explore within this mirror city and discover what has drawn it from its distant realm. There isn’t much time to spare, as the bug folk find their mammalian counterparts unnatural and blasphemous, and plan to destroy them.
This ring keeps soul in the body even when the wearer is killed, making bringing life back to the wearer much easier. However, if it takes too long, the wearer will begin to rot and become undead. Removing the ring causes the spirit to pass on immediately.
The runoff from Mount "Evil Volcano" has turned an area of small lakes below into a vision of hell. The mixture of acids in the lakes is of such strength that virtually any organic material touching is quickly dissolved. It is a melange of sulfuric, nitric and hydrochloric acids.
However, the place does harbor life, and its very strange, and very dangerous....
An enhanted purse that provides money, but at the cost of luck..
The PCs are accosted in a major city containing at least one famous fortuneteller / prophet of the future. They are informed that their as-yet-unborn child will (insert terrible evil), and that, although they are very sorry, the PC must be executed to keep this from happening.
Culture/Religion: based on fear and respect. Gods are very dangerous creatures, sometimes friendly, often not. Temples are the way to make contact with them... if not easier, then at least more concentrated in one place. Were it not for the temples, gods could be running amok among the people. Therefore, mortals have to keep the gods close to temples, entertained and worshipped. It doesn't make the bad ones any friendlier, though (and is no guarantee some won't go on trips now and then). Still, there have to be priests that are hardy men, able to survive the rigours of their position, get a sufficient number of worshippers to make the gods feel important enough, and mediate the contact between mortals and immortals.
In the Middle Ages, and even up to the early twentieth century, most of Europe's executioners were related: the Sansons and Deiblers in France, the Pierrepoints in England, etc. The reason for this was that, it generally not being socially acceptable to, well, kill people, executioners and their children could, generally, only marry other executioners or their children.
The parallels with massively inbred, Hapsburg-style dynasties are obvious- imagine a rather clever but politically inept satirist noting this, and being sentenced by the latter to a meeting with the former; even worse, imagine a dynasty of deranged and deformed executioners- think Texas Chaisaw Massacre with government funding.
The idea of using tattoos to contain magic powers is not a new one. The Ink gauntlet follows the basic premise of using rare and precious inks to inscribe spells into the skin of a mage or would be spell-imbued person. Some of these inked spells might be permanent, while others might fade after being used once, or a preset number of times.
In the days of old, before the dominance of humanity, the giants were the supreme rulers of the world and their crafts were considered to be the best. These beings venerated the god of the forge above all others and their swords and armors were the best that could be had even in the days of their decline. A hero seeking a masterwork sword might have to voyage long and hard to find a surviving giant smith or cache of rare and valuable giantcraft weapons.
Pick up a pathology/microbiology book, and add the diseases/syndroms to your game. Your players will freak out.
Coughed at? Diphteria. or tuberculosis?
Drunk from a muddy stream? Or just walked through? Well, let us introduce you to schistosomiasis...
Hit by a stream of energy? Too moch mana is ust like too much radioactivity - perhaps all mages could develop cancer early if too powerful...