Here is some information and ideas I have found:
I bought Disney mini fans, stuffed characters, Disney markers and crayons, mini Disney water game, stickers, light chasers, glow necklaces and bracelets, big Mickey lollipops and more junk. I'm wrapping them in Disney paper and leaving them each night from Tinkerbelle.
My kids are teens and they still like their surprises!
First night (we'll be going to MK in the morning) -- Mickey will leave pin lanyards (that I got from Kelloggs cereal for around $4 each several months ago)
Second night (we'll be going to Animal Kingdom the next morning) -- cheap disposable cameras left by Puma and Timon to take pictures of the animals
Third night (we'll be going to MGM in the morning) -- glow sticks & glow bracelets that they can use in the dark rides (like Tower of Terror) and at Fantasmic, left by ?? (Havent decided yet)
Fourth night (well be going to EPCOT in the morning) -- the Goofy scavenger hunt booklets that you can get free at the gate to EPCOT and Mickey Ear pens to write in them (I'll pop over and grab them one evening without the kids) left by Goofy
More Ideas
New pajamas with favorite character
Create a letter from Mickey, Minnie, Donald and Pluto. Download the Disney, Mickey, and Minnie fonts. Sign the letter with each characters font.