Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [name of person],
I would like to thank you for the exceptional service you have provided to us over the past year. We are very pleased with your [dog care/house cleaning/lawn care/babysitting/etc] work during the year, and look forward to continuing to do business with you in the year to come.
As a token of our appreciation, we have enclosed a small bonus for such quality and dependable services. It is very hard to find this level of affordable service these days, and we felt that you should be rewarded for all the extra work and effort you put forward.
Thank you again,