Non-obvious skills with highly measurable progress?

This seems likely to be controversial but I want to put forward "sales". Every so often I wonder if I should spend several months in a job like selling cars, where things are presumably really stark, but so far I've generally ended up doing something more kosher and traditionally "geeky" like data science.

However, before I knew a marketable programming language I had a two separate "terrible college jobs" that polished a lot of stuff pretty fast: (1) signature gathering for ballot measures and (2) door to door campaigning for an environmental group.

Signature gathering was way way better than door-to-door, both financially and educationally. Part of that is probably simply because there were hundreds of opportunities per hour at peak periods, but part of that might have been that I was hired by a guy who traveled around doing it full time, and so he had spent longer slower cycles leveling up on training people to train people to gather signatures :-P

Dual N-back

A Lung improvement machine $11

Sprinting

See The First 20 Hours: How to Learn Anything . . . Fast! by Josh Kaufman, whom I met at a CFAR workshop.

Math textbooks - you read the chapter, learn how to solve the problems, then attempt them and see if your answer matches the ones in the back of the book. Having a professor give you tests and grade them is optional but often useful.

Does it have to be something where the progress is easily quantifiable, or merely very easily observed? Learning to program is something that has an extremely long tail of improvement, but the first part goes very quickly. It may not be easy to quantify your progress - rough metrics like "lines of code to solve this problem" or "the amount of computer time / memory needed by this program" do exist, but they are at best only approximations of skill - but if you look back at code you wrote even a month ago (I'm assuming a not-too-strenuous instructional course, here) you'll be shocked by how quickly you improve. If you already know how to code, the same effect (and much of the same benefit) can be obtained by branching out into a really new language, ideally with a totally different coding paradigm; if you know C++, Java, and Python, try learning a functional language like Haskell of F#. If you've never worked "close to the metal", try picking up C and then C++. If you've only ever written application code, learn some scripting language(s) and write little tools to do things you currently do with a GUI file manager / config file editor / registry editor.

Always keep track of what you've written before (if you're doing this as part of a course, old assignments will suffice) and then go back later and write it again "correctly" using your current knowledge. It's entirely possibly you'll see obvious improvements week over week. Heck, when I'm learning a new language if I write 200 lines of code in it without going back and editing more than strictly necessary, the bottom half will be visibly better than the top.

Juggling

Touch typing

Martial Arts (if you count fluid movement and strength and such as measurable progress, rather than viewing the goal to be "combat effectiveness" or somesuch)

Speedcubing. I don't recommend it, though—I started about a year ago and it sniped a significant amount of my free time in 2014, on the order of 400-500 hours. (I had a similar experience with Go in college.)

Learning a foreign language.

Debatable how measurable progress in meditation feels, but when I've stuck with it in the past, I've subjectively felt noticeable progress.

Playing musical instruments, although this may or may not be "obvious".

For programing there are a few quick feedback programs:

• code.org for beginners

• codility lessons for advanced stuff

I would say consuming mass amounts of media, learning an instrument, talking with other people, and spending time observing.

Most of the less desirable skills have highly measurable progress and tight feedback loops. Picking fights, public intoxication, alienating friends with boorish slurs, etc. Perhaps something worth doing can be known in part because it does not come easily.