Each afterlife realm has a hidden treasure. Find these by piecing together the riddle fragments on each regions stelae. Or, you could jsut read this guide- while you need the stelae for the riddles, the treasures will still be there either way.
Treasure of Nefertiti
After reading the four stelae in Aaru (At the Gate of Aaru, Nefertiti's Spring, The Leaning Statue, and the House of Libations), find the treasure in a small hole where the shadow from the Tower of Deathless Feathers terminates.
Smash through the wood to open the white chest here, and retrieve your reward.
Treasure of Akhenaton
Once you've reviewed the four stelae in Aten, seek out the treasure in the library building at the north end of the Chambers of Reflection. Look in the northeast corner of the room, and note that the bookcase doesn't quite meet the wall.
Squeeze through the crack to reveal a secret area, with a white chest containing the treasure.
Treasure of Ramsses
This riddle (acquired by reading all the stelae in Heb Sed) is a little more literal than some of the others. Look for the Coward's tracks in a northeasterly part of the Kadesh Battlefield, almost directly between the markers for the battlefield and the nearby Scorpion's Lair.
Follow the tracks to a crack in the north side of the rock.
Watch for snakes.
Loot the Coward's corpse to gain the treasure.
Treasure of Tutankhamun
The location referred to is the Funeral Parlor. If you stand on the lotussy back balcony of the place, you'll see a spot on the river gorge wall with a five-pillar shadow.
Where the fifth (or rightmost) shadow falls is a crack in the rock.
Slide through, then vault over the miscellaneous supplies to the south east.
Make your way through these rooms to a large white chest which contains the treasure.