Simply walking barefoot everywhere is one way to toughen your feet. If you live in a city, spend time outdoors in your yard, on the concrete of your garage, patio and on the grass of your lawn. But there's more than just going barefoot that will harden your feet. Jumping rope, running, jumping...you need friction and impact. But, even walking barefoot indoors helps.
Wearing good backpacking or heavy duty hiking boots under load toughens your feet from friction and impact. You build up pretty good callous on your feet if you regularly backpack. Or you're an infantryman in the military. I've had my feet so tough while an infantryman that dragging a Cold Steel Recon Tanto I could shave with across my feet, would only cut callous. I have gone hunting barefoot at times, even in late Fall or late Winter, and there's something to be said for having access to sand, be it on a lake, beach or in a sandbox so that you can walk through it. Toughening the bottom of your feet is a result of impact and friction over time, but being able to sink your feet in sand helps toughen the sides and tops a tiny bit. You usually won't notice such things unless you purposely walk in nature barefoot, because you just don't encounter briars, sticks, rocks and other things contacting the tops of your feet on manicured lawns and pavement.
I've known an old farmer who soaked his feet in kerosene for prolonged periods so that his feet stopped sweating. He did it for about 20-30 minutes a day for a month, and he said his feet never sweated again. Essentially killed off his sweat glands I assume. But at the end of the day, soaked from sweating in the fields, I watched him pull off his boots and his socks/feet were bone dry...while I who was visiting, and from just short leisurely walking, would have socks damp with sweat on those humid Kentucky days. He had tough feet too. I won't pretend to know the medical wisdom or doing such a thing or not, but it worked for him and he learned it from others who had done it before him.
When your feet are damp, it's harder for them to callous up. I and many other infantryman have watched our hard earned callouses waste away and fall off in the swamps of Panama, where days of constant submergence or dampness were normal operating conditions.