How about you list some of the ways that they have helped you and some of the growth you have experienced through your work? How about a card or even letter stating these?

I was sent T a thank you card that said:

Thank you for always being kind, comparing and compassionate.
Thank you for showing me it is okay to cry.
Thank you for always having a safe office for me to explore and discuss my pain..

Having worked together for 10 years it was a pretty long list. She said it was very touching and meant a lot to her.