How to write a good luck/thank you letter to your...?

head of school who is retiring? I don't kow her well. I like her and she's nice and always says hi to me and know everyone in school's name (it's amazing!) but I don't know what to write her. I am writting a good luck/thanks retirement letter thing but don't know what to say. It's my mom's idea. Any suggestions for what to write? Thanks :)