Informal letters to friends and family can be written any time, for any reason. Remember to use everyday language, to engage the recipient with questions, and to express the way you feel through your words and your letter is sure to be a success.
Sending an informal letter to a friend seems to only exist in the forms of birthday and Christmas cards anymore. A less widely known evolution, however, is the meme.
Informal letter essay to friend » Daily Mom
A possible letter task in the CAE is an informal letter to a friend recommending (or not) a course/activity/holiday you have been on. Below is an example: