Independent frontier maths research, i.e. coming up with and proving (preferably numerous) significant new theorems without human guidance.
I say that not because I think the task is special among human behaviours. I think the mental faculties that mathematicians use to do such research are qualitatively the same ones all humans use in a wide range of behaviours that AI struggles to emulate.
I say it because it's both achievable (in principle, if LLMs can indeed think like humans) and verifiable. Achievable because it can be viewed as a pure text generation task and verifiable because we have well-established, robust ways of establishing the veracity, novelty and significance of mathematical claims.
It needs to be frontier research maths because that requires genuinely novel insights. I don't consider tasks like IMO questions a substitute as they involve extremely well trodden areas of maths so the possibility of an answer being reachable without new insight (by interpolating/recombining from vast training data) can't be excluded.
If this happens I will change my view on whether LLMs think like humans. Currently I don't think they do.