Math book present for a smart 8-year-old?

I thought I would buy some Christmas presents for my 8-year-old niece but I'm having a hard time figuring out what to buy her.

Do you have any ideas? Say, I'd use a budget of around $200.

I thought of giving her books to get ahead in mathematics and physics which will be useful for her in the future but I think would like to give her something that could be more accessible.