Probably my most stimulus-free time is when I am on the subway and my phone is out of batteries. I will eventually start thinking about random things, and let my thoughts drift... and sometimes I'll end up thinking about the most random things.
My thoughts do drift during the day, but rarely do they travel as far as they do when I am completely devoid of mental stimulation. I remember when Charlie was a baby, I would bottle feed him at night and I would sit in the dark and just think about my life and where it was going. It was actually a really magical experience.
I've started to block out times that I purposefully avoid stimulation and let my thoughts drift. What I've been learning is that my willpower is very weak. To truly avoid stimulation, I need to be in situations where I can't use any device. So sometimes I will leave my smartphone at home... or I was actually thinking of creating an app called Dumbphone, where if you turn it on you can't use your phone for an hour except for phone calls.
Do you ever experience stimulus-free time any more?