A treasure map leads the treasure hunters to their next clue.
This is a traditional treasure hunt clue - treasure hunters are given a map where an X marks the spot of the treasure.
The level of details added to the map is the key here, as that is how you can control the difficulty of the clue. Picking out certain "landmarks" such as distinctive room shapes, furniture layouts, location of trees etc is the best way to create the map, rather then attempting to draw the entire thing.
For a clue that is hidden outdoors you could mark the location on a map printed from Google Maps, provide that you are lucky enough to live in an area with high-resolution satellite images.