With 100 hours of investment, you can acquire a rare, valuable skill.
What could you add to your quiver that lets you bring more to the table?
Add knowledge of design principles, code, electricity, guitar or Scandinavian building techniques.
Even a skill that seems common could be uncommon in your line of work.
If you pick something off the beaten path, you might be able to become a leading expert rather quickly, perhaps in the history of noodles. Combine two things that are rarely combined and you have a shot at being the best in the world.
Since everyone is so busy working, hardly anyone is doing this. Those who do end up with something of real value for the rest of their lives.