Not that I disagree with the benefits of a long walk but driving in a car really helps in my opinion because of the white noise (http://en.wikipedia.org/wiki/White_noise#Applications), general silence with closed windows and change of scenery.

I don't know if white noise has really been proven to help concentrate but some people believe that. Alone in the car I've come with interresting thought and came up with good problem processing.

Showers are supposed to produced the same kind of environment (white noise and isolation from much of the external stimulus).