Confessions of a Homeschooler has a Mail Carrier Unit that includes the hat above, the activities pictured below and some others as well.this video that describes what a typical day for a mail carrier could be like and issues that they may have to deal with. This topic is also a nice time to talk about writing letters, your address, and what it takes to mail a letter.
My son helped write a letter for someone and put it in the envelope. Then when it was stamped and ready to go he dropped it in our mail drop box.
My son hasn't felt so great the last few days so even though there is a lot more you could do with this fun theme I kept it basic and he had a good time. Another idea I think is fun is to is to have children sort mail and set up a little post office. They could sort letters by color or shape or number.