In all seriousness, this is like 90% of ecotherapy practice. Getting outside has a number of measurable physiological effects that can also boost your mood. Your blood pressure drops, your resting heart rate drops, and the exercise boosts endorphins. Plus stress chemicals in the brain like cortisol drop and your sympathetic nervous system quiets down while ramping up your parasympathetic nervous system, and you get a nice dose of serotonin and dopamine. Outdoor activity can help with symptoms of depression and anxiety, as well as help with overall stress relief and fighting fatigue. If you have memory issues, to include due to depression, being outside can improve working memory.

And here’s the thing–some of these effects can kick in as quickly as after ten minutes outside. You don’t necessarily have to be hiking or doing something else physically strenuous; sitting and enjoying what’s around you also gets you some of these benefits. If you’re unable to get outside, having a window with a view of a natural scene is beneficial, and if all else fails, even pictures of nature can help. (Florence Williams’ book The Nature Fix has a lot more information on the benefits of being out in nature, and a good solid bibliography for even more reading.)

Obviously this requires a person to have access to greenspace of some sort, and the time and ability to get to it. But if you are able to get outside, whether hiking or walking or just sitting in a nice spot, it can really help achieve the wonderful “everything is marvelous!” feeling the OP is describing (I know it very well myself!)