I agree with borntohula, often times the most deep contemplation occurs to me while listening to music on a walk or a bus ride, and it's often the music I'm listening to that sets the tone for my thoughts. I would also add:
-long car rides with friends, because then you get those memorable sing-alongs
-live, because, well... ya.
-curled up with the ole significant other, because it's just so damn peaceful.