"Son, thank you for the fish. I'll enjoy it even more knowing that you were thinking of me while having a good time there.
Next time you are in this situation, think more about your responsibility as a guest. Your presence should enrich the experience of your friend and the host family, just as their invitation enriched yours. Show that you are having a good time, and your happiness will brighten their day.
We may be poorer material-wise, but in spirit, we are equal. They are very generous, not because they are rich, but because they have a generous spirit.
When you are there, you represent yourself. Focus on what you need to do. I know I am always in your heart, even without the fish."