Hello,
You may write Appreciation letter on the lines as below -
Dear .................
You will be completing 10 years on ............... (date) in this organization.
The Management would like to convey special appreciation towards your invaluable contribution so far. Your efforts and dedication has always helped our company in achieving the goals.
As a token of appreciation and such a long association, a small gift from the company ........................ *
We indeed look forward to your continued support in meeting company goals in future as well.
With best wishes!!
Thank you.
Yours sincerely,
Director **
* [Suggest you, not only a letter but also a gift should be there like silver or gold article as per company's budget]
** [A Director has to sign this letter to make it special for the person]