Attacked by a scouting party of Harpies far from their normal hunting grounds. They see the party, are hungry and go for it. If your party beats them off and manages to kill the one with the green hair and the silver pouch hanging from her neck they will find a map of the country scaled to show where your objective is and the nearest visible landmarks from their position so they can get their bearings. Assuming anybody in the group has the ability to read a map. If they kill 2 harpies the 2nd will have 2 whole days rations in her old scruffy smelly bag for 6 meals total.
No treasure because the map should be enough but if they can use the bows and arrows of the harpies you could make them stolen bows of master work quality. Number of Harpies small so as not to overwhelm the party but tough enough to make them fight hard.
Just a suggestion.