@remi The most valuable thing I've done is to learn how to learn new things.
The second most valuable is learning to work collaboratively as part of a team. We can't have expertise in everything we touch, which means we need teamwork and coordination.
Third, for me, is doing fairly deep dives into other fields that have some transferable overlap with the work we do - mathematics, linguistics, event management, and historical dance are some of the ones I've used a lot.