I have a goal of writing a thank you note a month to people who have helped me. (Reminds me, I need to do that this month.)
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.
I do a similar thing, except I email random people whose content or materials have helped me, or I appreciate.
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.
A colleague did this to me some years ago, he thanked me for the mentorship and confidence I gave him that enabled him to apply for a better paying job title.
It felt like rays of sunshine that came at a time were I particularly needed some, it made my week and then some.
Wow, thank you! this is really impressive! It feels like receiving a surprise ('dividend') from a past good deed. I will start doing this!