I suppose this applies to people who think of "retirement" as being sedentary and not challenging your brain. We need a word for what people in my social group

I suppose this applies to people who think of "retirement" as being sedentary and not challenging your brain. We need a word for what people in my social group who no longer have to collect a paycheck do, which is embark on things that either teach a new skill, have low probability of payback, or are intellectually challenging for the fun of it. It isn't "work" in that you don't have to put in time at someone else's business and sign over all your work product to them, but it isn't casual leisure either.