This is a wonderful way to add some Christmas magic to the season. My kids are a bit older (7 and 10) and so I had fun with it, I made scavenger hunts, or left clues. When I went on a business trip I mailed letters. The story the letters tell is fun and engaging and the kids wanted to see how it turned out every day.
International Elf Service
Making Reading FUN! ... Personalised Mail from the Most Magical Post Office