I have found that the best way to clear my mind and reduce stress is to get into the woods. The further away from civilization, the better. But even if I have to walk on an inner city trail, I get the benefit. During the last few years, I have begun to think that an added (or new primary) benefit has been getting away from all the technology in modern life. I do carry a cell phone when I hike, though it is often a hollow gesture due to lack of cell phone service. I do like to record my hike on a gps and do so with an app on my phone called Trimble Outdoors.
I have also noticed that the more adverse conditions the better. Rain and heat are the main forms of adversity, though one time I climbed Old Speck with a migraine (I don't recommend that, I was in tears by the time I got back to the parking lot). I have thought that the harder I have to pay attention to my surroundings and/or my body, the further I get away from all the modern "conveniences" that annoy me so much.
This was all personal experience and completely anecdotal, but now there is science to prove the benefits of time in the wild. One of my new favorite blogs has an article that discusses a study about this very topic: http://guthook.blogspot.com/2012/10/disconnected.html.
Read the article, it's not that long. Think about what it is saying. And then, turn the computer, TV, e-reader, phone, video game, etc off, pull the earbuds out of your ears and get out into the woods. Listen to the sounds (listen around the sounds of civilization if you can't get far enough away from it), feels the air and the earth beneath you and smell the smells of nature. Breathe. And then breathe deeper. After five or ten minutes you may notice that stress leaves your body with every exhale. Let your mind run wild. At first, it may seem like you are thinking about ever-present life. That's okay. Don't hold it back. Eventually though, you may find that your mind has slowed to what I call God's speed. I consider this a form of walking meditation. You may not even notice it at first, but I believe the benefits are provided regardless. Personally, I find this only when I am walking in the woods. Walking on the road, or even worse a treadmill (I know that sometimes this is the only option, but what a dumb invention) doesn't provide the same benefit.
There are many very active land conservation and trail groups out there today. You can probably find some woods near you in which to walk. I will admit, that some of the inner city trails are a bit scary, and you might want to stay off them, but there are others (I was on one earlier this week that went passed a tent city. I actually turned around when I came to a couple people smoking crack - I don't really know it was crack, but let's go with that.)
When you get done, notice if there is any benefit for you. Even if there isn't, do it again. And again. And enjoy.