One of the most mentally productive times of my life so far was when I had a 1 hour (each way) commute in a car with no CD player. I was alone in my thoughts for an hour a day with absolutely 0 distractions. Driving was just enough to keep my brain awake, but could get a low enough thread-priority to let me think.

I've replaced the driving with biking (I live a lot closer to my job now), but it honestly isn't quite as good. Some of the stuff I came up with then (this was about a year and a half ago), I'm still fleshing out now.

Another thing that's close to driving, but not quite as good is yard work. Something about the sound of lawnmowers and weed whackers is really conducive to thinking.