Have a look at https://arcprize.org/
They have hundreds of challenges that humans can solve in under a minute which LLMs can not. Seems the general trend is figuring out the rules or patterns of the challenge when there are few examples and no instructions.
Perhaps coding exercises that require 2d or 3d thinking, or similar. This is where I have seen LLMs struggle a lot. There are probably other areas too.
Ah, it also needs to be challenging for humans. It's a prize to win something. I just didn't want people to throw the question into Claude Code.