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.

Also just stretching, attempting balance, dance drills, etc.

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.