[ms-menardi]

Could solve Project Euler problems in Lua - aka, the easiest programming language to learn

https://projecteuler.net/

Alternatively, you could get a homeschool math textbook. They're written differently because the assumption is that the kid is going to have to teach themselves, and as such they are significantly more thorough and easy to understand. I highly recommend them. Don't get the kind that are "workbooks", those are usually trash. Find a good textbook that the kid can write in their own notebook for and sell after they're done with it. They'll learn how Ebay works, and algebra!