Hi there,
I'm just starting out on freelancer and your project caught my eye. I thought what a fine way to start, something short and simple, fun yet challenging. Additionally, writing mails and short letters are my forte.
I am open to suggestions /guidelines but if given a freehand, following would be the general plan
1) I begin by saying the obvious that you're moving on, continue on to say how hard a decision it has been for you.
2) about the time you spent and how it was memorable.
3) mention about your future plans
4) something that you learned there and would like to advise others about
5)Thank everyone in general and someone in particular, if need be.
6) How everyone can stay in touch with you and that you look forward to a lifetime of friends.
7) sayonara
That is the general outline of how the mail would be composed. And, additionally, based on information from you i would add some incidents that you remember, add humour and make it a fun but a memorable letter.
PS: Thank you for the consideration. :)