I found a website called Happier Human. It's about how to become and stay happier. I've trawled through it. Here are the best posts in my opinion:

[Meditate]. Don't [worry/overthink/fantasise/compare]. [Disregard desire]. [Motivate]. [Exercise gratitude]. [Don’t have kids].

[Buy many small gifts]. [Trade some happiness for productivity]. [Set] [happiness goals]

If you've found any other happiness interventions on any website, please share them.