According to a study in the American Chemical Society's journal, Environmental Science & Technology, just five minutes outside in a green space can improve mental health and wellbeing, and can decrease the chance of mental illness.
"From an analysis of 1,252 people (of different ages, genders and mental health status) drawn from ten existing studies in the United Kingdom, the authors were able to show that activity in the presence of nature led to mental and physical health improvements."
Any and every natural environment offered benefits, even parks in the middle of a city. The best combination was being outside in a green space that was also near water. The greatest increase in mental health and wellbeing happened in young people and those with mental illness. People were engaged in many types of activities, some for pleasure, some for exercise, some for work. The common element for every case was that it was outside in a green space.
http://www.sciencedaily.com/releases/2010/05/100502080414.htm