Or the PC's could be required to swipe something that was stolen and hidden amount the valuables of the Caravan. The reason it needs to be stolen back is because it would cause panic or an uprising if it were lost. Say the scepter belonging to the King and whoever has it can lead the people or something. Thus, they would be hired and would have to somehow sneak into the most guarded areas of the Caravan in order to get it. Then, perhaps that is when the ambush occurs forcing the PC's to decide if they will help the guards or not. In the end, have the item carried off by the raiders and taken into a large underground location where the raiders live. If they were orcs or something that might be a city of sorts. At any rate, the pc's would then have to make their way into the city and get the scepter back...maybe by then the King of the Orcs would be using the scepter himself and bending the clans to his will. Huh, maybe the Scepter adds a big Charisma bonus or something.

Anyway, you can keep adding on to this if we want to.