The folks I've done this for were pleasantly surprised, both by the note and the contents of it. They'd forgotten how they helped me, which makes sense, because it was a big deal for me but not for them.
A physical note, reminding them of the help they gave you, is a great way to say thanks.
Everything from professors, musical artists, authors, and professionals.
I never expect to get anything back but it's surprising how often I do.
My thoughts are that, if I did/wrote something that improved someone's life, even a one-word "thanks" would feel really good.
It felt like rays of sunshine that came at a time were I particularly needed some, it made my week and then some.
Also, paying it forward is the best way to give back and can create long lasting positive ripples.
Think about what someone did for you in the past (an intro, a kind word, a helpful convo an investment, whatever it was) and try to do it for someone in your life today.