Appropriate responses to unexpected gift?
Since you are using the reference "dong gua tong" (冬瓜湯, white gourd soup), I assume the restaurant owner is a Cantonese speaker.

The following are some commonly used responses to express your gratefulness from receiving a gift in Cantonese/Mandarin. The last two are not quite suitable in your case. They are generally used when you receive something which is bought by others as presents.


乜咁客氣呀! / 你太客氣了!


That's very generous of you!

多謝晒! / 謝謝你!


Thank you!

乜咁破費呀, 多謝晒! / 要你破費真不好意思, 謝謝啦!


Thanks so much for spending money on buying this gift for me!

多謝晒! 我好中意呢份禮物呀! / 謝謝! 我好喜歡這份禮物!


Thanks! I really like this gift so much!





Note

In general, the difference between "唔該" and "多謝" in Cantonese can be summarized as follows:


唔該


Used to express your thankfulness towards someone who did something to help you out.

多謝


Used to express your thankfulness towards someone who offered you something without requesting anything in return.



The aforementioned difference in Mandarin is not as obvious as it does in Cantonese. You may use "謝謝" in both situations. The phrase "勞煩你了" or "麻煩你了" in Mandarin is similar to "唔該" in Cantonese. "勞煩你了"/"麻煩你了" can be literally translated as "Have you troubled/bothered".