More people need to realize how fulfilling it feels to walk... it's such a great way to exercise your body & mind simultaneously. Feel your tension melt away, stop making a to-do list, listen to the birds... IN OTHER WORDS, be mindful.
I prefer to walk outside with the temperature right around 70 degrees. [this dream is hardly ever the case]
On occasion, I take my ipod with me on these walks, but really, I like to just take in the fresh air and breath deep.