For her, I think the best gift to put in a box has to have some certain characteristics. Such as sincerety, appericiation, honesty, and love.
For sincerety, I would definitely go for something that will demonstrate how thankful you are for being in her life, and you can do this with a handcrafted golden necklace that will shine her name wherever she goes. It will definitely be one of the best gifts that will complete her smile. Also, wherever she goes, she will take me with her with that necklace also!
For appericiation, surely it has to be something big that will make her feel appreciated like a wooden name sign that she can put on her room. Just seeing her with name on the wall will surely make her feel appreciated in her house and make her feel like she is a part of something valuable and that her being is appreciated.
When it comes to being honest, I want to be at her side all the time, whenever or wherever she goes, and I can do that with a personalized leather keychain that she will carry with herself wherever she goes. By giving a personalized leather keychain saying, "drive safe, I want you back," I know that I am being fully honest towards her with all my heart and that I demonstrate it throughly.
Last but not least, there is nothing can be done without love. For me, she deserves something valuable and that she can use, like a personalized leather wallet with flowers embossing. She can use it in her social or work life and whenever she uses it, my love for her will show itself because I know that whenever she uses it, she will remember me.
So, what to put in a gift box for her? Anything she deserves and likes, but one thing is for sure and that you need to think throughly for it.