I think what you really mean is how can one naturally have visions. DMT = visions. Speculating on what part of the brain produces DMT and what activities produce DMT is pretty futile. Because we really dont know a whole lot of that.
So basically dreams is one. Getting into dream recall and possibly lucid dreaming too. And the other activity where people have reported visions is Meditation. The yoga sutras of Patanjali, a meditation manual, even mentions encountering celestial beings while in a meditative trance.
But the sutras advice someone to ignore the celestial beings and not go into their world. This is chapter 3 verse 52.