About activities that promote happiness: flow

In his seminal work, Flow: The Psychology of Optimal Experience, Csíkszentmihályi outlines his theory that people are most happy when they are in a state of flow— a state of concentration or complete absorption with the activity at hand and the situation. It is a state in which people are so involved in an activity that nothing else seems to matter (Csikszentmihalyi,1990). The idea of flow is identical to the feeling of being in the zone or in the groove. The flow state is an optimal state of intrinsic motivation, where the person is fully immersed in what he or she is doing. This is a feeling everyone has at times, characterized by a feeling of great absorption, engagement, fulfillment, and skill—and during which temporal concerns (time, food, ego-self, etc.) are typically ignored.
In an interview with Wired magazine, Csíkszentmihályi described flow as "being completely involved in an activity for its own sake. The ego falls away. Time flies. Every action, movement, and thought follows inevitably from the previous one, like playing jazz. Your whole being is involved, and you're using your skills to the utmost."

To achieve a flow state, a balance must be struck between the challenge of the task and the skill of the performer. If the task is too easy or too difficult, flow cannot occur. Both skill level and challenge level must be matched and high; if skill and challenge are low and matched, then apathy results.

The flow state also implies a kind of focused attention, and indeed, it been noted that mindfulness, meditation, yoga, the Alexander Technique, and martial arts seem to improve a person's capacity for flow. Among other benefits, all of these activities train and improve attention.

In short, flow could be described as a state where attention, motivation, and the situation meet, resulting in a kind of productive harmony or feedback.