While it was drying we made the letters. I used various odd envelopes, and we used stickers and crayons to decorate.
We made a letter for everyone in the house, plus one which Harry decided was for "People".
Then when the postbox was dry Harry could post the letters.
I don't think that these letters will last very long, so I might make some more up and laminate them (I love my laminator!) to make them a bit sturdier. When Harry is a bit bigger there is a lot of potential to develope this activity. I love the blog post on ikatbag where they have made not only a general postbox, but personalised individual ones for each member of the family! There are also loads of ideas on this blog for other things to make out of cardboard boxes.