Anything you want!
Words, quotes, songs, doodles, jokes, stickers...the choice is yours! Whatever makes you think of that person, or you think they might like is perfect.
If you feel uncertain or even a little worried about what to write try to remember that receiving an actual paper letter in the mail is always interesting in and of itself! You can write as little or as much as you want inside and the recipient will still know they matter to you because they have something you actually wrote on with your own hand. It's very special. If you aren't confident expressing yourself in words try to keep it simple. Even a brief "This card made me think of you" is perfect and enough.
Happy correspondences!
Joy
