with older children you could do a visit to the main post office maybe phone them and ask if they do a tour to show kids who the post is processed and sent a trip to the post office is another way you could get the kids to write their own letters let them buy the stamps and then take them to a mail/post box if you wanted to could send the letter to a family member of friend or even back to your own address talk about your address talk about sending letter abroad and how they get a air mail sticker and travel via aeroplane they could even make their own post cards
