Many of these have been listed but here's my list :
There is a show in front of the castle featuring many of the princesses. After the show, they do autographs.
MGM - The Voyage of the Little Mermaid. A show w/ live actors. No autographs.
Mickey's Philharmagic - has 3D animated scenes featuring Jasmine and Ariel.
Ariels Grotto - Ariel is here, I think always, to meet.
The judges tent in Mickey's toontown tends to have Cinderella.
Jasmine is usually around the Aladdin Carpets in Adventureland in the MK. She is also in Morroco in Epcot a lot.
Alice and Mary Poppins are at the 1900 park fare breakfast.
Snow White is usually in Fantasyland near the old skyway. She is also in Germany in epcot alot.
Cinderella's breakfast - I think you know about this. Cindy is sometimes, but not often, in the castle too greeting guests.
The princess Storybook Breakfast at Norway's Askerhus is another princess breakfast.
Sleeping Beauty has been seen in Epcot (I forget which country)
Mulan has been seen in China in Epcot.
Beauty and the Beast show in MGM - live show, no autographs.
The princessess do appear in the Fantasmic show.
The parades, obviously.
Cinderella's dinner at Grand Floridian.
Esmeralda and Cinderella have been seen in France in Epcot.
Snow White's Scary adventure in Magic Kingdom (might be too scary for little ones though)
Pocahontas and Jane have been seen at Animal Kingdom. Both have shows and sometimes are in line for meet and greet.
I saw Megara at Italy in Epcot, once, several years ago.
Tinkerbell can be "woken up" at Tinkerbell's Treasures. It's not a live actress though, it's like a flicker of light (I think). I have not actually done it but have witnessed it. They let the first girl who asks wake her up.
Tinkerbell also flies over the castle during Wishes on good nights.
Once upon a Toy in DTD has a sort of Princess toy castle. It's just a display really but kids can get inside it and I'll admit , I went in it and thought "If I was like 7 , I'd want this in my bedroom!". It's a fixture in the store though, and they sell princess dolls.
I think you can also make your own princess hat here as well. For a fee.
I have seen Wendy ONCE in fantasyland.
I have seen Lilo at MGM doing meet and greet. Saw Megara there once too.
Oh - not sure if they count but there is a big statue of Lady and Tramp at Tony's town Sq. restaraunt in Magic Kingdom.
Nala does not appear anywhere that I know of , but we just told my neice that the lioness on the safari was Nala and she seemed quite happy about it.
That's about all I can think of.
Have a great trip!
