Well, I'll be honest. I did not read all of the posts, so if I'm repeating, please excuse my ignorance.
Here are a list of some of my personal favorites:
1. The key under the mat at Muppet*Vision 3D.
2. The wedding ring in the pavement outside the HM. If you can't find it, just ask a CM.
3. The Main Street Windows, for us Walt buffs.
4. The Brown Derby. Not a secret, but the most magical part of my trips. They often give me fastpasses for rides that have run out, i.e. Toy Story Mania, or other little surprises, followed by a magical, "You're in Hollywood, baby."
5. The phone booths around Epcot, stop and answer one. Or, for those of you at home, call one and chat with a stranger.
6. As you enter the MK, there is a hat store all the way to right, adjacent to Tony's. There is a phone is the back of the shop. Pick it up.
7. The paw prints outside Tony's.
8. The well outside of Epic.
There are many more that I have, but the best piece of advice I can give is this: Even though there is a ton to do, and not enough time to do it all, just stand around. Something will happen. A street show, a song, a dance, anything! Something magical will happen.