Non-obvious skills with highly measurable progress?

[robot-dreams]

A lot of my significant personal improvement happened as a result of highly measurable progress and tight feedback loops. For example:

• Project Euler

• Go (the game has a very accurate ranking system)

• Strength training

However, these are somewhat obvious examples, and I feel like it would be a waste not to push such a useful improvement mechanism as far as possible.

What are some non-obvious examples of skills with highly measurable progress and tight feedback loops?