How can I repay the kindness of my japanese friend and his family ?

A very good japanese friend of mine hosted me for two months (one in summer 24 and one in summer 25). Him and his familly went out of their way for me. For a total of two months they spoiled me and led me on adventures. I understand that this is a very VERY rare thing in Japan (and other countries, actually). I cannot stress enough how much they did for me. In return I hosted them for a month in my country.

Problem is ; They can't come back for a second round for reasons (work). Thus, I have no opportunity to give them as much as they gave me.

What should I do ? They know I'm thankful, is that enough ? What would you guys do in my shoes ? Knowing I'll live in Japan in two years bacause of the company I work in, I'll have ample time to see them again. I plan on thanking them properly during this time. But how ? If you have any ideas on how to react to such kindness, without being too "in your face" please help me ! Especially since giving presents is so important, but also has so many rules.

Thanks !