[Retric]

You can become a medic in a few months which is not a doctor but still able to save peoples lives in a wide range of circumstances. As to musical instruments learning to play the bugle call takes a while but not decades. You can learn some basic piano songs to play at parity's fairly rapidly assuming a reasonable level of dedication.

As for math even something as simple as being able to make change quickly is both useful and in demand.

[Mgccl]

I guess it depend on how one sees something to have value. There are mathematicians only consider research level mathematics have value. There are some subfield of mathematics like algebraic geometry that take years for a math grad student to even understand the language, and takes even longer to produce research paper with value.