Posting things is a very common schema/urge in young children, as is transporting, so our postal play session satisfied both needs - T-Bird could collect up our "letters" in her postman satchel, and then pop them through the slot in the postbox. Simple stuff, but she took it very seriously and was kept engrossed for a good old while.
T-Bird is currently unable to resist posting things at the moment, and so it seemed only fitting that I dug out G-Man's old, handmade and now-slightly-tatty, postbox for her to play with.
When I orginally made the box for G-Man, I happened to have the perfect shape and size box for the job, so most of the transformation was a simple paint job (red, of course!). I added black trim to the bottom, cut a flap for the letters and fixed on a little "collection times" notice frame. Finally, I cut out and added on the words 'Post Box' and a 'G' (for G-Man) with a crown, painted the same red to make them look like the embossing on a real postbox.
Both kids post all kind of things into the box, but current and most post-y things are the simple letters I made (cut out of thick white card and decorated with vaguely stamp and address-like squiggles), and our supply of Bear yoyo postcards. Those are absolutely perfect for this kind of play, and as the kind and lovely Bear folk have made PDFs of all of them available here to print out, now that promotion is over, our stock of these will never run out!