A cave with weapons sunken into the walls, blades sticking out of the floor and ceiling, and so on. Possibly, any weapon used in it could be torn out of the hand and sink into the stone.
A low lying area will flood after rains causing many people to be stranded and many roads are covered with water and unuseable.
Religious city. Hundreds of vendors selling small statues which sole value is as a donation to one of the many gods that are worshipped.
Minor Cultural Modifier: The culture believes, for the soul to leave the body, the corpse' eyes must be open. So any member of that culture takes care any dead body they find has its eyes open. Doubly true for those they kill themselves, for then the corpse will surely rise as an undead hunting him.
In cases where a person has accused another of a crime, but there are no other supporters of the accusation or defenders of the accused, and there is not enough evidence to support one party or the other, the two people involved, or their champions in some cases, must duel in the presence of the local ruler or leader. The loser is assumed to be lying and punished accordingly.
There is an important pair of fraternal twins. Possibly minor dieties, sorcerers, or maybe just an important pair of nobles. One is evil and one is good. The thing is, the one who looks evil at first glance is the good one, and vice versa.
There is a group that believes no good can come of trying to change to future to render a prophecy or prediction incorrect. They believe that meddling will either cause the prediction to be true in the first place, or make something worse or more extreme happen.
There is a person who has a well-known reputation, probably involving something illegal. But this person is either very clever and crafty, or really like his or her privacy, for no one knows who this person actually is. As an added twist, perhaps this person is of an age that would noramlly be wildly inappropriate (say, a child is a much-feared bandit) or maybe not the expected gender or even race.
Other improvisational weapons/tools:
Icicles are useful as daggers,
Bag Of Holding filled with copper pieces used by chucking the contents at foes, (by strolen)
sleeves make useful garrotes. Ya can't cut anyone's neck, but you sure can choke 'em!
Flaming Logs make great clubs, (any Barbarians out there?)
and Bee hives are very effective when thrown at a mob of enemies. :-)
The Chinese, when attacking a castle or fort, flew kites over the city wall and used the length of string it took to get it there as a measurement to know how far they had to dig a tunnel to get under the wall.
A culture has a tradition of wearing animal pelts as a sign of status or job. Carpenters might wear beaver skins, Masons have a moleskin hood to their cloak, Gaurdsmen might have badger pelts. Done to show the culture's respect for nature and how much of nature is equal to each other.
Termites, in certain places, make homes that can be eight meters high and only a half meter wide. They are built facing North to South to take advantages of the suns travel, maximum heat in the morning and evening and little in the afternoon. Imagine grassy plains with vertical structions facing a certain direction all over the place. Ambush? Maze? New creature? Larger structures?
Silk Armor? The Mongols wore silk undergarments under layers of leather armor. Why? Because silk is very strong. If an arrow hit them and made it through their armor, it would usually not have enough energy to puncture the silk. It could still enter their body but, because the silk would not break, the arrow's barbs could not do their work and the arrow could easily be removed leaving a relatively clean wound considering normal arrow wounds.
Perhaps divinations, if strong enough, can actually change the future, instead of predicting it.
A possible answer to what happens to spells when a mage dies. If the spell is strong enough, say and enchantment or other permenant effect, part of the mages spirit may become lodged in the magic. It may be a way for items to gain some kind of intelligence, but a mage who has knowledge of this fact would be very hesitant about enchanting anyone or thing. He might have other plans for his afterlife than counting the change in your bag of holding.
Preists, I think, would have this sort of thing covered.
A magician develops a new way to make scrolls and can sell more powerful spells for cheap. Problem is, whether the magician is aware of it or not, the spell's power comes from spirits trapped by the magic that makes the scroll. Once used to power the scroll, the spirit is driven mad by the forces that have ripped through it's being, and often develops a homicidal thirst to destroy the one who tormented it. The spell the spirit was used for may have left some residual power in the spirit to give it more abilities than it ever used to have.
A new substance is found in an out of the way area and kept very quiet. It is found to make a perfect counterfeit diamond. Only the most expert of expert appraisers would even have a hope of noticing that it is fake. Only one problem: if it gets wet it starts to slime and if it is saturated, it turns into a glob of gel.
If a creature is transformed into another creature (say, a human into a dog), then that creature will become more and more accustomed to the new form until a point is reached where reversing the spell is impossible. A second transformation is possible, but the creature will have to go through the whole process again.
A certain type of demon cannot not be hit by ranged weapons or attacks. Attacks have to be made up close and personal for the damage to mean something. Ranged attacks are to impersonal.
Possibally a way to make the ranged attacks more meaningful would be to coat the arrow head or what not with the shooters blood. Of course, they'd better be a good shot, otherwise they're wasting arrows and already bleeding to boot.
Spells: wizards might have half or no effect, preist might work due to divine intention.
Weapons or equipment that is heavily relied on can be "named". Then the equipment begins to gain abilities beyond those of normal equipment. They might siphon off some of the experiances of their owners (1 to 5%) and level up on thier own. Could be an unintenitional way of creating artifacts. Ships could become sturdier or seem to just barely outrun the worse of a storm that would have surely sunk another vessle, swords could fumble less or resist dulling more, a farmers plow could turn stones aside easier. Anything that is depended on as much as an inividual can depend on as much as another individual could be "named".
"It is said that among those people they have a loathsome custom- that they keep a spotted dog always waiting beside the gate of the yard where they bury the dead, and that in every funeral they allow this spotted dog to feast upon the dead, so that it grows fat and wise with the knowledge of the dead... Many necromancers do seek out these spotted dogs, and ask of them sciomantic knowledge, or take them as familiars." -Author unknown, "The Ways of the Necromancers"