Okay, I thought of some more:
Look at the centre window on the first floor of Phantom Manor and you just might spot the Bride behind the curtains. This is pretty much impossible to see on a bright day, but when it gets dark a light will occasionally illuminate the window for a few seconds. Also look out for the Phantom lurking behind the furthest left window on the first floor on the side facing Boot Hill. Again, a light will come on behind the window every once in a while.
You can also hear the sounds of a dentist performing a particularly painful operation on a patient, a man shaving and various other noises coming from the upper floors of certain buildings on Main Street USA.
On the waterfall lift in Pirates Of The Caribbean, look above you and you will notice lots of little glowing eyes peeking down at you. These eyes are also to be found later in the skeleton caves and in the first tunnel in Big Thunder Mountain.
In Alice's Curious Labyrinth, the Cheshire Cat is more than just a head. The hedges that grow overhead to the right actually form the stripes of the cat's body, as seen in this sattelite photo:

When riding Big Thunder Mountain, after the second lift hill, just after coming out of the big helix you go through a canyon before dropping down through a small tunnel. Just before you drop, look above and you'll notice a pack of animatronic coyotes (two to the right, one ahead) It's surprising how many people miss this. Oh, and in the final tunnel, after the third lift hill, look above you to see a group of glowing bats screeching at you.