A study by Taylor and Kuo (2009), has found that time spent outside can help reduce the impact of ADHD symptoms. Not only that, but park environments seem to have more an impact that city settings.
Seventeen children with professionally-diagnosed ADHD, ages 7 to 12, experienced three different treatments. Two of the treatments consisted of a 20-minute guided walk in the city (one downtown, one in a neighborhood), while one treatment was a 20-minute guided walk through a city park. Each walk was done one week apart, and the order of the walks was randomized.
Children with ADHD had more improved scores of attention after walking in the park, compared to scores after both city walks. Raters were blinded as to what walk the children had experienced.
I've noticed in my practice that people with ADHD, regardless of age, tend to have an affinity for nature and the outdoors.