Lets see... my best known Disney Secrets (Not the Vacation Club!)
Magic Kingdom: A amazing restaurant is hidden in Main Street USA called the
Plaza Restaurant - It's a Sandwich place that has really high quality food. They take reservations, but a lot of people don't know about it, so walk ins are usual available. To get to it you take a right immediately past the bakery, as if you were cutting through to Tomorrowland and there it is.
EPCOT: One of the best places to get rare pins is here. Of course it depends on who is working the PIN Station near the Fountain, but the cast members there always have amazing or rare pins on their lanyards.
Another helpful hit - There are only 3-4 visible public bathrooms in Future World, but there are way more than that.
Bathrooms that are always busy:
East and West Blocks in Future world - East Block is that locations ONLY bathroom. No really. Only one bathroom for Test Track, Missions: Space, and Universe of Energy.
West Block used to be never used much, but Soarin' came and changed that.
The two bathrooms near Spaceship Earth - Always busy due to traffic walking through.
Some unused bathrooms:
Behind Journey into Imagination - There's one sign, just one showing it's back there. No
Electric Umbrella (This isn't used very much, trust me.)
The Living Seas Bathroom (Not used much either, people don't even know they have some inside either.)
The Corel Reef - This is a public restroom that anyone can get to as it's in the lobby.
Hollywood Studios:
To be honest - I don't know much about this park. It's been changing so much and it's the park I visit the least.
Animal Kingdom:
A lot of people don't know that there is a Single Rider line for Expedition: Everest. If you don't mind sitting away from your friends, it cuts the wait in line by 90% in my opinion.
This will allow people to get on Kilimanjaro's Safari early in the morning and not have to worry about using a fast pass on EE as well.
Keep in mind though, sometimes the Single Rider line is not open however.