I have been thinking about this.... I have this standing desk, so I wonder if it would make my soles tougher if I would gather some nasty looking rocks to stand on when using that desk.
Not too nasty or you'll start finding excuses to avoid your desk. Maybe start with trainer gravel and work up the grades list until you exceed your comfort level. At that point, dial it back a notch and you'll have the ideal working / plantar development surface. You might need to contain the material in something like a litter box or a kiddie pool or it will migrate throughout your house.