The level of math amenable to apps isn't really what Su is talking about.

I would suggest learning proofs, and maybe pick up some Art of Problem Solving books. Or perhaps working through the foundational curriculum of any decent math program (e.g., algebra, analysis, topology, number theory, etc.).

Khan Academy can get you pretty far, if you like going through the remedial stuff then you can even take the quizzes to get "Mastery" in various subjects like Algebra, Trig, I think Calculus too. But I'd love something else that is more adaptive and gives you exercises and education as you go. As a hobbyist one of the hardest part is figuring out where to jump in so you're neither bored nor overwhelmed.

I'm sure an app would defeat the purpose...

Euclidea is fun. I'm not sure if that's the sort of use you mean.