Most times we cause of most the slowing and reversing in the growth of our mind. We trick ourselves into thinking being exposed to more things/information is same as growing our mind. It's true that travelling and putting yourself along the path of new experiences expands the mind. The trouble is that abandoning all the mental tasks you used to do when younger contracts your mind. You've stopped trying to memorize hymns and now find it hard remembering things you read. You've stopped reading and are now constantly experiencing tip of the tongue syndrome. Your brain is already shrinking, and maybe faster than travelling is expanding it.
You don't have to give up one for the other. You don't have to let your brain shrink in some ways and expand in other ways. As much as you can, you should try to keep every mental skill you have. If you are good at memorizing things, then keep at it. Learn a new language. Memorize new words, language rules and sentence formation in another language. It's not compulsory that you keep doing the tasks that got you that originally got you the skill. You don't have to keep memorizing arithmetic table or the periodic table. Find something more exciting for your new stage in life. If don't feel any motivation to learn a new language, you could learn a new field of study that will help improve your life. Like personal finance. It will involve you learning new words too: asset, liability, budget, cash flow, annuity, retirement savings account, risk-free rate, risk premium, interest rate, compound interest and many more. And you will be better for it, financially.
If you played sports while in secondary school and the university, but now work in a bank with little time for leisurely activities. You still have Sundays. After church make it a goal to do one of your favourite sporting activity, even if it's for just 20 mins. Don't let the part of your brain that developed to make you good at that sport begin to atrophy. And if you are like me whose only major sport while in the university was walking to reading room and class, things could be better now. Learn a new sport. If you are rich enough join a golf club. If you are on the way to getting rich, like me, then learn a cheap sport. Live a life that is just as active or even more active than the one you lived at school.
Most importantly, never stop learning hard tasks. Now, there are no lecturers or exams to force you to learn hard things; it's up to you to push yourself to your current learning limits and stretch those limits. Don't depend on your children to operate that new gadget for you; learn about it yourself. Read all you can about operating it. Don't keep pushing all the unfamiliar tasks to your junior colleagues; break a sweat as often as you can. Always be in the know.
Travel wide. Read wide. Learn wide. Exercise well. Keep memorizing. Take up very hard tasks. Feed your curiosity. Don't let your mind age faster than your body. Grow your mind.