Modern hippy hipster types who actually accomplish tasks of great magnitude and worth. A happener, much like our modern definition of a hipster, does have certain interests in counter culture lifestyle and the various accouterments that go along with such. The marked difference between a hipster (being frankly the modern version of the hippie or renaissance person), is that happeners are always involved in some great events. Happeners are the hero's of the universe. May statues be built in their honor.
Did you see how intensely that woman back there was involved with her artwork, she was a total happener.