Some activities need little mental effort such as playing certain video games. With practice, you improve at them. The same idea applies to useful skills like typing or solving maths problems, which have practical benefits. What other activities could someone do that are either helpful for an engineer or generally useful and need little thought?
This isn't quite what you're looking for, but you could label data. It's low effort (usually) and creates something useful, although it doesn't necessarily benefit you personally.
Walking or bicycling (or whatever mobility mode one can do), on the low-effort end, without sound in ears or eyes on screen; this lets the mind wander and eventually, interesting ideas and syntheses bubble up.
That's a good one. I was thinking more along the lines of something low effort that I could do at a desk or on a couch that will still lead me to improving a skill of some sort.
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