My girls get letters every 7 to 10 days. I bought stickers of Disney characters, some glitter pens, and colored ink pens. I chose a character such as Ariel or Mickey. I write a letter from that character and use the character's sticker as a signature. Then I use glitter glue (pixie dust) to make a border around the letters. I make them at work during my lunch hour. Then when I pick up the mail at home, I mix the letters in with our day's mail.
For envelopes, I fully address them and use a glitter pen to make a "stamp" with a Mickey head in it so they immediately recognize that it is from "Disney World."
For the letter from Cinderella and Pooh, I will invite them to have breakfast with me. ( We are doing their character breakfasts.) The letter from Belle will be to invite them to her Storytime. The letters from Mickey, Minnie, and Donald will be to "stop by my place at the Toontown Fair."