We're all players in a game called evolution. This means we're biologically designed to cope with adversity, not success. The real contest in life is between closely matched organisms, each having only a small chance to survive.
Being a creature that's 1% more or less appropriate to its environment (not too smart or dull, not too strong or weak) is the actual game. Everything else is random noise. This means becoming spectacularly successful makes you irrelevant -- a spectator.
This can lead to artificial contests, between organisms that have already won the primary contest. So, after saying, "Wait, what now?", people invent make-believe goals -- climb mountains, make even more money, learn to cook a perfect omelette -- hoping to restore a sense of purpose.
In this non-contest, the least successful burn out, maybe even die prematurely. The most successful invent an imaginary, artificial goal that turns into a real advance. Transistors. Lasers. Antibiotics. Things that weren't really necessary until they appeared and, by existing, became their own reason to exist.
So ... done climbing mountains? Choose an important, unsolved problem and work on it. The problem list is long and deserving:
Batteries.
Cancer.
Population.
Late-stage capitalism.
People who want to kill everyone who doesn't share their beliefs.
Not necessarily in that order.