In my opinion for a person that has it "pretty good" objectively, the limiting factor on their happiness is what the Default Mode Network (DMN) is (or is not) doing. 

I think that at least part of it, can be roughly equated with what we consider the ego to be. It involves a lot of thinking about the past, planning the future, thinking about other people - but always in a self-referential mode. If the person is stressed and unhappy then the DMN is spending a lot of time ^[1](and the brain a lot of energy) on

• ruminating about past events
• producing anxiety about future events
• evaluating one's relationships with others (how one was hurt by others, how one hurt others) and doing status comparison with other people (how one does not measure up for example)

What I think has the best potential of significantly increasing their happiness (actually the happiness of each human being) would be a transformative change in their relationship to themself. What we might call Self-Transcendence.

A couple of most promising interventions:

• Psychedelics (5MeO-DMT^[2], psilocybin^[3]) which can very reliably produce a mystical-type experience. A majority of people in ^[3]cite this experience as one of the 5 most personally and spiritually meaningful experience in their life (and a significant portion as the single most meaningful). In a mystical experience, the person is able to dissasociate from what we could call their 'ego' and reevaluate their life. It's also associated with a sense of oneness and connection (which occures naturally when the ego is downregulated and its barriers lowered)
• A 10-day meditation retreat such as Vipassana can be incredibly valuable for the given time investment.
  • If you could get your hands on the most promising mindfulness enhancing brain stimulation technology developed at SEMA Lab (that I'm aware of) that could really change things for the better (here's their promo video by the Guardian - with a very clickbaity title)

1. ^{^}

   I think it's important that none of us actually want the brain to be spending energy on this stuff. I am aware of gratitude journaling but not of "fear journaling", "jealousy journaling", "inadequacy journaling" etc. 

2. ^{^}

   A single inhalation of vapor from dried toad secretion containing 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT) in a naturalistic setting is related to sustained enhancement of satisfaction with life, mindfulness-related capacities, and a decrement of psychopathological symptoms

3. ^{^}

   Mystical-type experiences occasioned by psilocybin mediate the attribution of personal meaning and spiritual significance 14 months later
    

Antidepressants do actually seem to work, and I think it's weird that people forget/neglect this. See Scott's review here and more recent writeup. Those are both on SSRIs, there is also Wellbutrin (see Robert Wiblin's personal experience with it here) and at least a few other fairly promising pharmacological treatments.

I would also read the relevant Lorien Psych articles and classic SSC posts on depression treatments and anxiety treatments.

Since you asked for the meta-approach: I think the key is to stick with each thing long enough to see if it works, but also do actually move on and try other things. 

Meta: Generic questions like these often get very different and much better answers when one talks about specifics. [rule of thumb: make the question specific enough that the answer would only help you and it would help nobody else]