This leads to "Buried Tweasure":
Maru sends you to pick up an Ancient Treasure Note, located in the Ruins of Uldum, which is in the Valley of Watchers.
/way #71 37.61, 84.12 Ancient Treasure Note 1
Nearby, you'll see two Ancient Toys which give you coordinates to go to. These are optional, as you can just skip ahead to the next note for actual % completed. Some people like using the Thinking Cap, though.
/way #71 37.28, 82.92 Ancient Toy 1
/way #71 37.38, 82.75 Ancient Toy 2
They lead you to the northeast, and you'll find another Ancient Treasure Note next to a rock.
/way #71 47.51, 56.49 Ancient Treasure Note 2
Head to Zul'Farrak, outside of the instance, and find another Ancient Treasure Note next to the wall. You'll need to have your torch out for this one.
/way #71 38.84, 22.11 Ancient Treasure Note 3
This last note has you headed to Gadgetzan, and you'll find the "mysterious traveler", Yennu, in the Inn.
/way #71 52.69, 26.86 Yennu
Return to Maru for the reward!
Note: Technically, the note given to you by Maru is treasure note 1, but since you find the others, I've labeled them as 1, 2, etc.
Just waypoints:
/way #71 37.61, 84.12 Ancient Treasure Note 1
/way #71 47.51, 56.49 Ancient Treasure Note 2
/way #71 38.84, 22.11 Ancient Treasure Note 3
/way #71 52.69, 26.86 Yennu