walking, hiking, and road tripping are my favorite things to do (walk daily, "hike" (just long walk since I live in flat Houston) weekly, and roadtrip monthly). And the entire time, I sit in silence just THINKING.
Sometimes I record my thoughts out loud. But for the most time, I just use it as a way to keep my mind churning. Sometimes I come up with new side projects, or a design for a new piece of furniture, or just wonder on certain things (friends/family, state of the world, the guy that coughed on me at the restaurant).