What is a good birthday gift for a 12 year old who likes business?

Twelve is a genuinely good age for this, and the gift should match what a twelve year old can actually hold rather than what a sixteen year old could.

At twelve a kid can set a price, explain what they do, follow up with somebody who did not answer, and track what they earned against what they spent. What still sits with a parent is scheduling, anything involving a stranger's home, and every dollar that moves electronically. So the sweet spot is something they can run largely themselves, in a neighborhood where people already know them, without needing an adult in the middle of every transaction.

That rules a few things in and out.

Works well at twelve. Anything for neighbors who already know the family. Anything made and sold at a table with a parent nearby. Anything where the work is visible and the transaction is simple.

Harder at twelve. Businesses that depend on online selling, since the account and payment side needs an adult. Anything requiring power equipment. Anything inside somebody else's house.

Sophie is twelve in the book for exactly this reason. What she does needs no equipment, no license, and no permission beyond a ride, and the thing she learns is about buying rather than selling.

On the gift itself, the failure mode at this age is buying equipment for a business that does not exist yet. A twelve year old handed a kit of gear for an idea they have not tested feels watched, and the gear becomes a reminder of something they did not follow through on. Ideas and a first step beat equipment, every time, until somebody has actually paid them.

If you want to give something physical and useful, something that makes them look like they meant it works well at this age. Cards with a real number, a shirt, a sign that is not handwritten. Twelve year olds are taken far more seriously by adults than they expect once they look organized, and that first experience of being treated like a person running something changes how they carry it.