After you break the lens at the end of the game, look closely at Atrus. For a brief moment a face will appear. This also happens at the start of Riven, when Atrus holds up the Linking book to Riven. This was intentional, so that it felt like Gehn was watching you from the very beginning.
Hint: Hidden faces
At the start of the game, leave Temple Island and then return. Go to where you started. Next, go to the bottom of the staircase and look down to the right. Two faces are in the top-left. To find the other three, rotate the Gate Room (rotating room) until you can enter from the passage under the gate. Go under the gate, and immediately after you do that, on the right side of the screen, you will see three faces. You will only see them if you can enter the room from the place you are in.
For more hidden faces, step outside the cage where you start the game. Turn left, and look up. One face is just to the left of lower center, hidden in the rocks. The other is in the lower right corner.
More hidden faces can be seen on Temple Island, after going under the chained wood gate. The passage to Gateway room at other end must be open. Look along the right side of the screen. The three children's faces are Rand Miller's daughters.
Hint: Human
Go to the 233rd Age (Gehn's house) after trapping Gehn there. Go down the ladder to Gehn's bedroom. Go up to a sort of round cage thing (the "holo-projector") and look in the rocks in front of you through the window. There should be a human there.
Hint: Jungle secrets
When you change to Disc 3 you should be at a point where all the trees are cut down. Find the path that leads to the gate. Open it and start to walk. As soon as you go under the first tree, look to your left. You should see a giant dagger sticking up out of the ground. Walk down and you will find another rolling ball thing. Remember to note what the symbol is and what sound it makes. Return to the path and start walking forwards again. When you reach the two pathways, go right. As you walk down you should see a giant fish head. Approach and inspect the head and you will find nothing. However, back on the pathway you should see two lamps. Go until all you can see is the top part. On the very top there is a small disc. Push it and see what happens.
Hint: Name written in rock
Go to the workshop island (Disc 1). After solving the boiler puzzle, go to the small balcony that has the frog trap. Climb down the ladder. At the bottom, face the ladder so that you are one screen away from being directly at the ladder. In the upper left corner is the word "Alex" written in the rock.
Hint: Photo on cliff
In Riven, after you have advanced to a decent point in the game, go to the chamber where you first appeared in the game. In that area, go to the edge of the cliff facing the holograph room and the first transport. Look down the cliff and you will see rocks and the water. In the upper-left side of the screen, in the cliff, is a photograph of two "MYSTerious" children. Those are Robyn Miller's children.
Hint: The "deleted" book press
There was a book press on Crater Island (where Gehn makes his books), by the ladder up to his "house" (which was locked at the start). It was removed during the game's development, but still can be found. Face the boiler controls and click in the upper left hand corner, on the pump. You should now see an angled picture of the nice view to the left of the boiler. Look down towards the left side and you will see the upper part of the book press. Another way to see it is to walk a bit back on the catwalk and turn towards the boiler room. It is difficult to find, but it is there. If you look up, you will also be able to see it, just even more blurred.
Hint: Gehn's face...