Do you have any opinion on rote work, for example data entry, or stuff like scanning the incoming checks or mail for a small office, filing papers, etc.? I sometimes worry that this type of work isn't as much of a brain challenge, but maybe there are some people who don't mind it so much because it gives them time to think.
Actually, I think rote work is quite good too. Problem-solving on a high level is obviously engaging some part of your brain that rote work doesn't engage, but simply reading / writing / doing maths, even the simple fact of having work to do keeps one from vegetating. Thinking about other things while you're doing it is a bonus.
I wouldn't put manual labor into this category, though. I've got a minor obsession with reading memoirs by people who've survived extreme situations like polar expeditions, trench warfare, captivity, torture, etc. (Probably all started when I read Robinson Crusoe). All their stories seem to congregate around the idea that having to something to do with your mind is crucial to survival, even if it is somewhat rote.
