If you spent many years as an engineer solving problems and got really good at anticipating problems before they happened, then retirement requires the same type of mental stimulation to keep the creative juices flowing and ward off mental atrophy. Your brain doesn't suddenly shut off when you retire, for many it gets even more active because you are not constricted by your work. Many try a completely different area of interest, like artistic work of painting, sculpting, woodworking, etc.
I chose to work in my same field, building a plane for myself to fly around the country. Well, it was plenty of work and after 4 yrs of flying, it still requires almost daily work to make "improvements". I think you need a hobby that before going to sleep, you think and plan about tomorrow's activities. A plane gives such stimulation and maybe even too much of it.
I'm also doing more creative writing, nothing dramatic or special, just helping write newsletter articles for pilot's association and a few articles for trade magazines in composites. The point is to maintain the daily activities and get into new trials. So a couple of years ago, I took a class in clay sculpting and found out I was terrible at it. A lady next to me did something worthy of Michael Angeleo and I just didn't have the talent to turn an idea into a thing of beauty. But I can make good mechanical drawings and can machine parts that fly so you explore what you are good and bad at and pursue the good stuff.
Another thing; don't retire if you have to count pennies everyday. To enjoy retirement, you need sufficient funds to do spontaneous trips or attend classes or make things or do dinner out. There is little point in being retired but not being able to enjoy life. All the big money expenses like house mortgages, kids college costs, health issues, should be taken care of earlier so monthly expenses are minimized and more cash is available to play with. Or marry a rich person and disregard my advice.