Now, we all have heard about the RIVEN easter eggs, but what 
about Myst? I have to get nostalgic. Here are the two hidden 
images. The first is Atrus' note to Catherine, but it is crumpled
up. The second is the checker box found in Sirrus' room on 
Mechanical Age. The box is opened, it appears to be a music box.

The Crumpled Note