What do you get someone who already has all the Mathematica T-shirts and posters?
(Sadly, this link to the good stuff at the Wolfram store is almost impossible to find https://store.wolfram.com/view/misc/ )
Maybe they've already read A New Kind of Science? A somewhat lighter book that my kids enjoy is Adventures of a Computational Explorer by Stephen Wolfram. It's good for kids who are not overly serious, or for grownups who feel like they are kids.
They especially liked the one about the spikey, the Wolfram logo that has a bunch of math behind it.