Nearly every adventurer loves treasure. So much so that "kill the monster and take its loot" is the most common adventuring trope. Treasure is an expected part of almost every role-playing game. In games with more modern settings, treasure might be information or special equipment, but in old school fantasy rpg's, treasure meant coins, gems, jewelry, and magic items. After a while though, even the treasure can become boring if it always the same. . . . "The party finds X silver pieces and Y gold pieces. How do you split it up?" What follows are some ideas for making treasure a little bit more interesting and sometimes outright deadly.
In stories, the thing the hero quests after, which is often insignificant to the plot, is called a macguffin. For example, a party of good knights might be sent out by their king to find a holy relic, the mace of St. Cuthbert, and return it to their kingdom. One day the party locates the mace. The mace is a treasure, but not one the party gets to keep permanently. Do they keep it safe? Where do they keep it as they travel? How do they store it? Does someone use it? Is it cursed? Does the mace have powers the knights don't know about? Are evil creatures drawn to the mace or repelled by it? By these questions, you can begin to see how macguffin items can be used for more than just the reason for the trip in the first place.
Another type of treasure that characters often seek come in the form of social advancements. A social advancement is anything that changes the social class of the party. Here are some examples of social advancement being used as a treasure. The commoner hero rescues the princess and wins her hand in marriage! (What do you mean you do not want to marry Princess Grunzelda the Huge?) The brave heroes slay the beast and the king knights them! (Making a dwarf, a half-elf, and a tiefling Sacred Knights of the Holy Protector has caused a rift in the kingdom; some say the king has lost his mind.) The duke is so pleased with the group that he gives them 50 acres of land each, making them landed nobility. (After they swear fealty to him, of course. Plus, 50 acres containing 45 acres of treacherous swamp is hardly a prize, is it?) Having successfully completed the quest, the adventurers are welcomed into the Explorers Guild in a fancy public ceremony. (A faction of the guild members are aging aristocracy; they control the rules. Dues are 10,000 gp/year. Guild members must march in 3 parades each year. Junior guild members are not allowed to speak in meetings. Etc.) The previous examples should demonstrate how social advancements can be used as "double-edged" treasures.
A Part of a Multi-Part Item
Here is another example of a treasure that is a reward that promises a greater reward later. In this case, it is assumed that both the pieces and the whole item are magical in nature. If they aren't, then this is simply a multi-part macguffin, i.e. find all of the pieces of the broken vase. What makes this type of treasure unique is the power of the individual items, as well as the power(s) they have when combined. For example, let's say there is a four part staff that allows the wielder to summon and control undead. One part, functioning by itself, is a +2 club. When it is combined with another part, which allows the wielder to create skeletons, both parts lose their individual fuctions and become an item usable only by clerics that can control minor undead. The headpiece of the staff could be a luckstone that loses its properties when combined into the whole item. Most of the fun here comes from the lack of knowledge about what will happen when the items are combined. Perhaps once they are combined the items cannot be separated again without powerful magic or perhaps the item was meant to be useful both in parts and as a whole. It could be quite possible for a power good magic item to be a part of an evil artifact. It is quite possible that your party has already acquired a piece of a multi-part item and they don't even know it. Any gemstone, rod, or wand might easily be part of a larger magical construct.
Seriously, just how quiet is your group of adventurers? Maybe you are underground, where sound carries pretty well, and you're killing mooks and henchmen left and right. You know the Big Bad must be near. You burst into a large chamber and see the Big Bad standing before 5 treasure chests! Its henchmen rush to engage you. You get your slay on and the Big Bad quickly realizes that it is outmatched. It teleports away leaving you to kill off its remaining henchman and loot the treasure it left behind. The group revels in the vast treasure that the Big Bad has abandoned. At some point, perhaps when they get 5 miles away from the dungeon or perhaps not until they get all the way back to town, the adventurers are going to realize that the treasure is FAKE. The coins are all copper made to look more precious. The gems are common river rocks. The magic items nothing but temporary dweomers of power. Seriously?!? You didn't think Big Bad heard you coming? You really think you drove it away?!? You are idiots carrying Big Bad's tracking device that looks like a ruby. This demonstrates just one way that you could use counterfiet treasure.
Here is one of the most common ways that the party can acquire a treasure that isn't. Cursed items are sometimes found among normal items of treasure. Items that appear beneficial, but have only a negative modifier and can only be taken from the afflicted character with some sort of remove curse spell, are the most common form of cursed item. In this scenario, the PC is only affected by the curse until they can find a spellcaster to remove it. I prefer to muddy the waters when it comes to cursed items. If an item has both beneficial properties and detrimental ones, especially if the beneficial properties aid ONE player and the detrimental properties affect another or the group as a whole, then there is going to be a lot of interesting role-playing in your group upon whether or not to get rid of the "cursed" item. This is one way that you can used cursed items.
Another way doesn't really involve a curse at all, the curse comes in the item's uniqueness. Whether the Big Bad is using a locate object spell to find the +1 Cloak of Protection with a silver sunburst on it that used to belong to one of his lieutenants or just hiring assassins to go after the person who is wearing said cloak, you can see how the item is cursed without actually having been imbued with any magic to make it so.
Finally, we get to the worst of the treasures that aren't. Treasures that are NOT treasures at all, but traps to cause harm to the PC's. Here are some examples: an illusion of an overfull treasure chest hiding a pit trap; a small hoard of gold coins covered in a slow-acting contact poison (after they count them with bare hands, they make the save); a locked treasure chest that contains no treasure, only a glyph of lightning bolt activated when the chest is opened. Other examples of this include monsters like trappers and mimics that pretend to be inanimate objects and coins that are physically or magically "marked" (I'm sorry. I can't take that silver. It belongs to Grundarr the Slayer, see his mark. Do you have any other money?). Go easy with this one. PC's HATE being undone by their own greed.
Some Final Notes
Do not overuse these techniques. They are meant to be seasonings, not bread and butter.
Remember your goal. My goal is always to tell a good story, so I use techniques to that end.
Another form of "social advancement" could be access to a limited resource like the Sage's library.
These techniques are best used when mixed in with real treasure of various sorts.
Remember that everything the party does in public makes a statement to the townfolks.