I feel like this question is more about gift-giving in Japanese culture. But since there's the question of proper language involved, I am going to give it an amateurish stab. Japanese gifting etiquette is fairly unique and different from other cultures.
One common thing people say in Japan when giving gifts is to describe the gift as commonplace or trivial, as a humble gesture.
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> This is really nothing of value.
Some other options:
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> Just something trivial to represent my gratitude
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>
> Just a token of my gratitude
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> Just some thing trivial
If you want to mention the gift is a token of your appreciation for something the other party did for you in the past, you can say something like
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>
> ...
If you want it casual/informal
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