Others have posted the good rides & shows, so I'll give you something different. Here's some ideas for you to try that are unique things I never miss!
Magic Kingdom
-Take the girls to Cinderella's wishing well, and let them throw in a coin and make a wish. It's to the right of the castle if you're looking at it, up a pathway.
-Have one of them ride Cinderella's horse on the carousal. It's the only horse w/a gold ribbon on it's tail. If you ask the cast member working the ride, they will point it out for you. It makes you feel special =)
-Hunt for paintbrushes on Tom Sawyers Island. Around 6 are hidden everyday, and if you find one, turn it into the raft driver for a special prize. Be there as soon as the island opens, usually around 10:00 AM.
-Have each of them try to pull the Sword from the Stone, a few times a day it actually pulls out for someone with "A Special Touch" - fun for kids!
-Show them the Rags to Riches fountain in fantasyland. You can see the crown behind Cinderella on the mural will sit on her head when looked at from a child's angle.
Epcot Center
-Take them to a character dinner at the rotating restaurant "The Garden Grille". The characters come to the table and get pictures, the food is good, and it's fun for kids that the restaurant spins. (Make reservations)
-Don't miss Soarin! Head straight for that ride if you can, the line backs up. It's IMO, the best ride in Disney World.
-Get a times guide to see when Mikiyuki is making candy in Japan. She will let your girls pick an animal and will make candy that looks like that animal and they can keep it for free. It's pretty cool.
-Definitely stay for Illuminations, it's spectacular.
Hollywood Studios (MGM)
-The new Block Party Bash is an interactive parade, and I've not yet seen it, but it sounds really cool.
-When you do "Muppets 3d", look for a door that says "Key Under Mat" before you go into the show. If you lift the mat by the door, you will find the key. Sometimes there will be a magic certificate there congratulating you for finding the key but I think that might be only the first time everyday.
-In the back of the park is an umbrella connected to a pole, you can hold onto it, just like in "Singin In the Rain", and if done right, it will create rain around the umbrella.
-Pull the rope outside the Indiana Jones show, you get a funny response.
Those are some of my fun sort of secrets! I'm a big kid, so I get a kick outta these things! If you ride the monorail, ask to sit up front w/the conductor - great view! Have a great trip!
Jamie