Health, to a large extent. Everything is more fun and less stressful (to the mind and body) if you are somewhat fit. It feels good to feel your body work when you do something heavy. This also include things like hiking in a forest (or whatnot), trying out things like making a fire out in the woods. Try it, if you haven't already.
edit to add, it is by extension profitable to focus on your health, but in less obvious ways.
As a software engineer working 60% as well, I'm currently using my 40% for studying towards a degree in a non-tech field (social sciences). I'm glad that my country (Switzerland) has such low tuition fees which makes that kind of continuing education possible.