Put it in their envelope, include a sheet of stickers, seal it and address it to them. Put these in a small mailbox or cardboard box made to look like a mailbox. Have them go to the box and retrieve their mail. Read each of their notes to them. Talk about how much fun it is to get mail.
Some mail brings happy news, some brings sad.
Letters are an important way we keep in touch with each other. Give each child a sheet of stationary and an envelope. Encourage them to think of 1 - 3 things they want to tell someone. Write it on their stationary for them. Have them fold it up, stick it in the envelope and seal it shut.