Look at some of the earliest letters we have from the Roman fort at Vindolanda (at the British Museum website: tinyurl.comlvdzl or Vindolanda and Roman Army Museum site: www.vindolanda.com).
Ask pupils to create their own letters from the fort, before having a bit of fun burying them and digging them up a week later to see whether they can still be understood!
Ask students to imagine they are using the new Penny Post to describe the wonders (or horrors) of industrialisation to a country cousin.
Take the inspiration of Martin Luther King's open letter and write something similar about a different theme, even something which moves students today.
Look at key letters, especially forgeries. You can examine the Darnley question at www.learningcurve.gov.uksnapshotssnapshot02snapshot2.htm
or the 20th century Zinvoviev letter at www.learningcurve.gov.ukcoldwarG1cs2default.htm