They love to do this over and over. If you have a wagon or push cart have your child be the mailman and have him deliver your mail to the mailbox or to another member of the family. Make up little notes to your child and put them in the mailbox. They will love opening the letter and listening to you telling them what it says. You can also put the mail you get for them in your mailbox into theirs.
Young kids love to pretend to write letters even before they can read or write.
What a great way to teach your child to write thank you letters to grandma. Tell them they need to write a thank you note to Grandma. Let them doodle on the paper then write your interpretation of the note on the other side. Have your child put it in an envelope stamp it and mail it himself. Cut a flag shape out of red cardboard and fasten it to the oatmeal box with a paper fastener so that it can be moved up and down.