Answer by Alexander van Hattem:
As far as I am aware, none.
White matter is brain tissue that consists of the axons of nerve cells, as opposed to grey matter which is composed of the nerve cell bodies (called soma). Different kinds of nerve cells can have single or multiple axons, but to increase the amount of white matter the cells would need to produce additional axons beyond what they originally differentiated to have, and I don’t recall any evidence that this can occur to significant degrees.
Here some examples of different types of nerve cell:
What you may be thinking of are the studies about how some activities can change white matter organisation. The brain does change on a structural level based on experiences, but what changes seems to be the arrangement of the white matter rather than the amount of it.
Any activity in which you learn something new, or consolidate a learned skill, should result in structural changes somewhere. Which region of the brain you affect will depend on the nature of the skill, but also on which areas of your brain are not fully mylinated, a process which is ongoing until about your mid 20s.
Examples frominclude playing an instrument professionally, or learning to juggle. In principle many other skills would work just as well.