After two and a half years of being retired, moving to a warmer climate, playing golf, riding my motorcycle across country and doing anything I damn well please I have become brain bored.
You don't have to take on a job to keep the brain entertained.
I've always had a strong need for intellectual stimulation.
Early after ER taking a 3 semester ornithology home study course from Cornell University took care of some of that need for me. I got to do it on my own pace, but still cracked the books, turned in the exams, and got the exam scores back. I've since cracked a few other science texts - just for fun! (what a geek!)
In addition, I've ended up learning a HUGE amount of computer tools for video editing, graphic design, photo editing, etc. The tools are complex and endlessly evolving. They are quite demanding on the brain. The output of projects using these tools are unique to us and incredibly rewarding. I've produced a lot of cool "stuff" over the years. I sometimes fantasize that if I HAD to earn a living, I would do graphic design. That seems to give me the most immediate enjoyment. I can spend hours manipulating images.
My husband spends a HUGE amount of time programming. But this is to populate his giant database of all his photo data and automatically generate web pages for his photos. It's incredibly sophisticated. I tease him sometimes about how it's like he's back at work - especially with the late night programming sprees, but it's all for his own enjoyment, and to share those web sites with other naturalists, of course.
Over the years we've also developed expertise in bird watching, fine art printing, photography, ...... The list goes on and on.
Never occurred to me to work for someone else to keep my brain entertained....
The closest I got - I almost submitted a magazine article plus photographs to a couple of bird magazines. Then life got in the way - we got too involved in going fulltime RVing.
But sincerely - to each his own. Whatever floats your boat.
Audrey