Fresh air has long been touted as something able to cure whatever ails you. And it's true. If you make it a point to leave the office and step outside once every day, that can go a long way toward defusing any stress that's been building from your open office setting. So head out during your break for a walk around the block. Go out to lunch at your favorite restaurant, or eat your sack lunch on a park bench. Don't let inclement weather stop you; make sure you have boots, warm clothing, an umbrella — whatever you need to fight the elements in your area.
Remember to breathe deeply when you're outside, too, to get as much air into your lungs as possible. (Deep-breathing is also a stress-reliever.) Exercise helps boost your mood, too. If you're able, take a brisk walk or even run during your outdoor break, assuming your workplace has shower facilities. If none of this is possible, walk to another part of the building once a day. Simply leaving your surroundings for a while can provide stress relief and a sense of perspective.