I think the easiest way to lifelong happiness is cultivating curiosity and gratitude. I was lucky to have been born with curiosity, and over many years I learned gratitude.
Curiosity is outwardly-focused, and leads to being interested in other people, different opinions, new experiences, investigating problems, and enjoyment of the things around you. Gratitude leads you to be thankful, humble, considerate, empathetic of other people's situations, and happy with all the things you have and all the little joys in your life. Contribution leads you to notice opportunities to help, to add to the world, to be a producer instead of just a consumer, and ultimately to share what you have, what you've made, and what you know.
The opposite is chasing fame and wealth.
Chasing fame is inwardly-focused, with the quest being all about you, your appearance, your recognition, your ranking, and your perceived power over others. Chasing wealth is about more, acquiring, surplus, showiness, selfishness, and power over others. Both are also dependent on outward circumstances, and can be notoriously fickle and short-lived. They also promote the false premise of an end goal, a place where you are finally Rich and Famous. But as a path to happiness, it's a lie, because once you choose that path, you are never rich enough or famous enough.
Choose curiosity and gratitude, and you can be happy right in this moment. Add contribution, and you can be happy for the rest of your life.