What do you think is the highest leverage activity one can do?

Good question, I would think there should be some interesting variations.

For me it's probably experimentation & discovery.

Plus I would say the general trend is that the higher the leverage, the less likelihood of achieving its ultimate upside potential. This seems to hold true across different types of ambitions, not only financial, emotional, physical, interactional, etc.

"Finding compatible partners" seems underrated.

Developing deep relationships with friends, co-founders, coworkers, spouses, etc. is extremely fulfilling and productive :)

Organizing and reorganizing files into folders by type and first letter. Oh sorry, I thought you asked about the lowest leverage activity. Not that I ever do that.