I don't know if this has been postd before, but in Frontierland have you ever noticed that on the side of some paths there is yellow gravel? This is because in the "Frontier" times they would have urine troughs on the side of the main rodes for horses to urinate on, turning the gravel yellow.
Also, in Liberty Square the building's shutters aren't straight, they droop. This is because during the Revolution they took off all the lead hinges and made them into bullets for the war, so they secured the shutters with leather straps, which eventually stretch causing the shutters to droop.
In Mainstreet the buildings gradually get shorter, this has two affects. When entering the park, Main Street seems extremely long, adding to the mystique, but when you are tired and exiting the park, it makes Main Street seem smaller.
On the Space Ranger Spin, when you enter a room through a round door immediately spin around and imagine a hm, then shoot at the right ear, apparently it's the 1,000,000 point target, but I haven't tried this one yet.
Look at the hole in the top of the Splash Mountain from the OUTSIDE of the ride, look for a Side Profile of Mickey.
While on Splash Mountain, right before the big lift I think listen for the little creature that says FSU, and if you don't catch the "S" he is not yelling profanity.