I want to thank a mentor of mine who has encouraged me, helped me, and inspired me greatly. However, while I consider this person my friend to some degree, they are still my superior and it would be less awkward (and more practical) to thank them privately, like with a kind of small gift or letter.

But, I don't want to give them a really long letter detailing every little thing I'm grateful for, nor just give them a card that says "thank you," and I don't know what kind of gift I could get them to show my appreciation without imposing.

I also work along side this person and as their assistant, so maybe it would just be better to say thank you in person? How would I go about that?