When contemplating content for an application think of what the application’s intended purpose is from the perspective of the person evaluating the student. It is generally a means to evaluate the candidates ability to thrive academically and contribute to the schools community.
There will always be a presumption that the applicant is highlighting the most relevant, impactful and impressive skills or attributes they have to offer.
Is your child’s Rubics cube abilities significant within this context?
I know you are looking to find “interests” to list so I am not being dismissive just suggesting a way to think about it.