I don’t think there is any need to write anything related to One Piece. After all, this is just your girlfriend's hobby.
For example, I am a girl and I also like to watch One Piece. Of course I would be happy if I receive a One Piece-related gift from my boyfriend.
If there is a card in the gift, it will of course make you happier.
If it were me, I would hope that the card contains what my boyfriend wants to say to me (remember not to write unreliable words like forever and ever)
1. It should Write down the feeling of getting along with her, such as how long you have known her, and the happy and unhappy things you have done together, and review or express your own opinions. (I can’t teach you how to write this, after all, I’m not the person involved)
2. And you know she likes One Piece, so you bought this gift, hoping she will like it (of course she will definitely like it) La, hehe) This way she will feel that you are very attentive and observant
3. In addition, you can also write about what changes you hope she will make on her birthday or what minor problems she can get rid of (but this should be written Be more tactful)
4. Finally, add your blessing to her and express your love (be more practical!)
This way your girlfriend will receive you The card will definitely make you very happy, and you will feel very happy and warm in your heart.
I have divided it into paragraphs for you, which is basically the content.
I hope my answer can be helpful to you!
I hope it can be adopted.