We just returned from WDW with OUR 3 and a half year old dd. She loved:
MK
Carrousel
Many Adventures of Winnie the Pooh
Pooh's Playground
Mickey's Philharmagic
Snow White's Scary Adventures (she did it twice)
Dumbo
Meals - Crystal Palace and Cindy's Royal Table
Ride on the Train
Small World (FOUR TIMES! SHOOT ME!

)
Adventureland
Tiki Room
Aladdin's Flying Carpets
Tomorrowland
Indy Speedway
Tomorrowland Transit Authority
Liberty Square
Meal - Liberty Tree Tavern
Toontown
Meet and Greets
Mickey and Minnie's House
Goofy's Barnstormer
She also loved Wishes.
In MGM she liked Playhouse Disney, Muppet 3D, The Voyage of the Little Mermaid . She also LOVES Fantasmic.
In Epcot, all that intrested her was Illuminations and the fountains
Animal Kingdom
Festival of the Lion King
Pochohantas
Safari
Meals - Restaurantasaurus
Also, try and get an ADR for Chef Mickey's and a ride on the monorail. She LOVED both of them too.