The stuff that is hard changed the brains wiring. Studying logic, practicing rotating objects three dimensionally, mathematics, abstract thinking like classical philosophy, sophistry and politics, and even day to day social interaction with people with diverse personalities would be a good start I should think.
Have you been following the latest articles on keeping the mind active as a way to delay Alzheimer’s? I collect wherever I can and pass them on to friends. Things are so much more hopeful today than they were when Reagan was diagnosed. Not there yet, but there’s hope.