The contents of the letter depend on a lot of factors. What purpose are you going to use this letter for. Is it going to be for a job? or an academic endeavour? Maybe you are looking to make use of the good gesture of the department head to get a signed letter which you could use in future?
If you have a clear-cut purpose in your mind, put it in the question, and the content of the letter can be varied.
However, if you are trying to get a general purpose letter, you can keep the body short and simple. Make sure to include the nature of your work and day-to-day duty while on the job.
To whomsoever it may concern,
I have known Mark as an employee who lead the team in companies IT department. I found him to be diligent with his duties and always showing enthusiasm in his professional life. He has an innate desire to learn and grow as a professional.
I would certainly welcome any opportunity in the future if he wish to work with the organization again.
I have no doubt that he'll be will be a valuable addition to any team that he works with in future.
I wish his good luck and success in his future endeavours.
The above is just a starting point for a generic recommendation letter. Do not try to use template/copy-pasted content. Originally and simplicity trumps down adornments any given day. Advise you to keep it short and simple, such that it gets read in a short glance.