My personal favorite is one that cannot be seen by the general public any more. I have a picture of it that I'll have to scan.
It is located up on the hill behind Goofy's house...facing Fantasyland Theater. You used to be able to spot if from the theater until they revamped it.
When they did the concrete work for the rocks, they actually made a full 3-D Mickey face and even painted it! You had to sit in a certain section to be able to see it.
My second favorite is the "Where's Elvis?" book in Minnie's kitchen. From the story that I heard, with Mickey's Toontown, the Imagineers didn't want to hide MM since it's his homeland. Instead, they hid stuff about Elvis. I believe there are 3 Elvis items in Toontown.