How about the PC's are hired to garrison some piddly little outpost on the border with a neighboring kingdom when A) that neighboring kingdom decides to come through that area on a raid/conquest/other B) hostile monsters in the area attack the outpost, looking for loot/revenge/some item or artifact rumored to be somewhere in the outpost but never found by previous explorers. Catch: The obvious catch would be that whatever the situation, the PC's would have to be either high enough levels to face it, or have enough diversity of characters/terrain/options for magic.
How about the pc's are hired to pilfer a certain item, any item that could have a real or perceived value to the pc's or their employer. When they get there, the item is already being stolen or they've been set up and now must try to get out of the situation without losing face or their lives. Catch: Having a thief in the party would be really useful, but parties that don't have thieves can have and npc thief come along, which could make for a more interesting plot twist if it turns out the ambush is set up to catch only the thief, and the rest of the party shows up!
How about the pc's are implicated in the destruction of some item with real or perceived value and now must get out of town or find the real culprits before time runs out. Catch: Players can get disgruntled real quick if this happens more than once or twice.
How about the pc's, while sent on a mission (could be almost any) accidentally come across an item which takes them into a labyrinth or maze of some sort. For higher levels, the maze could be some sort of reality displacer, taking the through time at various points, or perhaps space. They exist at that point in time though they are not from it. However, they do not exist in that SPACE in time. They are still in the maze, and it'll take cunning to figure the trick out and get themselves home. Catch: Besides not being able to do this very often, a plain maze can bore players quickly. Add a minotaur or a Gorgon for flavor.
How about the pc's are hired to guard a caravan that carries a secret item which becomes subject to several raids and more subtle inter-caravan subterfuge. It may turn out that members of the caravan are also out to get the item. Catch: This has to be played for flavor. If the item has real value, make sure the players can only get their hands on it if it turns out