Last week was National Postcard Week, and there were crafters who made special postcards to swap during the week (see some NPW postcards here).
Maurice Sendak passed away last week, too, and one of the online tributes showed a picture of an envelope that Sendak decorated to send through the mail.
Looking for examples of decorated mail led to me discovering a type of craft work known as "Mail Art." Links to more about mail art can be found here.
Nick Bantock's, Griffin and Sabine: The Extraordinary Correspondence [F BAN] is a literary exploration of mail art and should provide inspiration for you to work on your own.
If you'd like to try your hand at mail art here are a few books that contain crafting ideas that you can adapt:
Boutin, Vicki. Creative Foundations: 40 Scrapbook and Mixed-Media Techniques to Build Your Artistic Toolbox. [745.593 BOU]
The Michaels Book of Paper Crafts. [745.54 MIC]
Ritchie, Judy. Great Rubber Stamping: Ideas, Tips & Techniques. [761 RIT]
Stevenson, Cheryl. The Art of Handmade Paper and Collage: Transforming the Ordinary into the Extraordinary. [751.4 STE]
Sendak envelope courtesy Letters of Note.