What is the best way to say "thanks" to my new friend?

So, I was sick for much of last week. My new friend is very kind and gave me substantial help on our project. He also kindly shared his notes with me, so I can get caught up.

I was thinking of giving him back something, but some kind people I’ve met in the past don’t really want anything in return for their kindness. I also don’t want to look like I’m trivializing everything into simple bargains and exchanges (which I am not trying to do in the first place).

Alternatively, I can do something nice for him in return but he is quite self-sufficient and rarely asks others for help. He is also rather quiet, so I don’t really know if there is anything he needs help on. I could ask, but he might just say that everything is ok.

Any ideas on how to say “thanks” to my new friend?

(BTW, the simple answer of just saying “Thank you” works, but it seems that some say thank you without really meaning it. I want him to know that I really mean it.)