If you want to see the MK fireworks and don't mind not seeing the castle go to fantasyland in front of Winnie the Pooh. You have a perfect view, and it is usually free of people.
Key under the Mat at the Muppets (3d) MGM Studios
Some have said it's not there anymore, I personally cannot verify this.
The rope in the well outside Indiana Jones Stunt Spectacular MGM Studios
Small door in the rock wall in the Boneyard at Animal Kingdom
Mice in the rafters at Cinderellas castle Magic Kingdom
Lady and the Tramp paw prints outside of tonys Magic Kingdom
Umbrella on the lamp post that drips 'rain' when you hang on it MGM Studios
If you have trouble finding it, it's easiest to find close to the entrance of Lights, Motors, Action! If you are standing at the entrance of LMA, look back onto the city streets and it's on a corner. It's hard to find if you don't have a general idea of where it is.
Talking and walking trashcan in Tomorrow land also see if you Magic Kingdom
can find the person who is making him walk and talk.
There is also one in the Electric Umbrella CS restaurant in Epcot. It's stationary but it still talks.
Devine (women dressed as trees/ on stilts) Animal Kingdom
Epcot Rose. That is located past the Fountain of Nations on the EPCOT
way to WS. It is very easy to never even notice it but look for the
pinkish red tiles as you walk toward WS, it is in the very center
Wait, what is this?
Whitewash Brushes on Tom Sawyer Island Magic Kingdom
There are days that they don't hide any paintbrushes on the island but you can ask the CM taking you over to the island on the raft if there are indeed brushes hidden that day.
There's an old wall phone in the candy shop on Main Street where Magic Kingdom you can listen in on a mother/daughter conversation.
There's also one in Tomorrowland. See picture below, it's located on the Tomorrowland Indy Speedway side of the Astro Orbiter.
Cinderellas horse has a gold bow on her tail at the carousel Magic Kingdom
you'll see a huge net hanging from the Ceiling...it's full of redish MGM Studios
squares and odd shapes...well it's full of Jello...as bad of jokes as
you can get...its "A-net Full-of-jello" get it...Annette Funicello ...
ehhehe...ooh.
When you're waiting in line next to the Haunted Mansion's front doors, read through all of the tombstone epitaphs. Pay special attention to that of Madame Leota. Every minute or so, her face presses out slightly, her eyes open and dart back and forth, and she returns to sleep. People will call you crazy for about a minute, when five other people see it and start shouting.
Also, a couple of other tips I love for the HM is look up when the stretching room gets dark... trust me. And, if you want to exit the stretching room first, go to the side of the room with the painting of the lady.
Once you go through the mansion, take your time exiting. Take a gander at the engraved wall of the mausoleum and the pet cemetery for a few chuckles, then see if you can find the granddaddy of Disney World scavenger hunt photos - the bride's ring in the sidewalk. Skeptics say that it was never intended as a secret, but was just a leftover from a pole that was removed. But Disney lovers and cast members have constructed quite a story about the ring, which they say the doomed bride pitched out the window before murdering her husband-to-be. It's embedded in the stone sidewalk and is very difficult to find, but it's there.
And here's the saddest one, per the 2007 HM refurb, the ring has since been removed. Rumor is that Imagineers REALLY didn't appreciate the story made up because of it since it really wasn't intended to be there so they had it removed in 2007.