Handwritten thank you card, and a tin of handmade cookies.
Of course, not a generic "gee thanks" kind of thank you card, but how much you really appreciate everything they do and why, etc.
Heartfelt.
Things like this are meaningful to the receiver, and remembered.
My daughter takes martial arts. That is what I do.
Per your other question, about hings they can improve... well, then have your child practice at home. Regularly.
For us, we sometimes video our daughter during her class (its fine with the Sensei and other parents) SO THAT, we can play it back at home and then 'recognize' what things she can or may need to work on.
And during her class anyway, I am THERE waiting and I watch the lesson, too. Myself.
So I do know... what areas she needs to work on. And the routines or Kata movements.
Occasionally I ask the Sensei how my daughter is doing. And then they let me know.
But no, they don't have sit down meetings with the parents about it. It is not a teacher conference.
It is an individual journey... but it also takes dedication and wanting to self-improve. Whether that is personally and internally or per their actual movements.
At my daughter's Dojo... ALL of the instructors/Sensei, are Volunteers. They do not, get paid. But yes, it is a business for the owner of the Dojo. But it is for the kids. And a meaningful business.