This is one of the easiest ways I have learnt to relax.
I often find that when I get engrossed in my work I can find myself sitting in my seat for hours. This is not good for me! Luckily (sort of?) my body does not respond well to hours of sitting still, so I naturally want to move around. I also find that my brain functions better when I have been out for some fresh air. I recognise this when I start to think in a loop or I feel 'stuck'.
Going for a walk outdoors is a great way to shift your state by moving your physiology - the stretching, movement, breathing and use of large muscles brings oxygen into the system and allows you to have a different perspective on something you might be working on.
I recently read that it is great to be able to take on the habits of smokers, without actually smoking - deep breathing and frequent breaks outside. Try it yourself and notice what difference it makes to your productivity.
In NLP terms, this is known as a 'break state' - where you change your physiology/environment in order to alter (and improve) how you feel. It is also a great way to reset your stress levels and refresh your body and mind!