Once at the post office, take the time to look around and point important things out to your child like the scales to weigh the packages, the special forms, the postal workers, the items other patrons are mailing, etc. It's also important to note the sequence of action since this will take place in your play -"First, we get our package ready to mail - then we pay for our postage - then the postal worker puts it in a bin for the mail carrier - then it gets delivered."
I'm not sure if I'm allowed to take photos inside a federal building, so don't tell on me. I'm also not sure if I'm allowed to take one of everything at their preparation desks for us to play with later, but I did anyway.
At home, set up a few stations, if you have the room. A mail preparation area, a postal worker area complete with scale and cash register, a mail box with a slot (the slot makes all the difference for kids) and a delivery area.
Here is a preparation area. Writing out the words that your child will use is a great early literacy tool. Even if your child can't write his/her letters, having a model is a great start to show letter formation and just the idea of word usage. I also like to use old stationary and envelopes to add to the mood, as well as stickers to use as stamps. A hole puncher is also a good additon since it works on building fine motor muscles and helps kids make holes (they love that).
Finally, it's delivery time!