Read, read and read.
Reading is one of the greatest stimulations for brain. Find articles which interest you.. Even a gossip column would help. But do not get hooked to anything. Change topics of reading. Stuff that leads to new Knowledge is the best, as it stimulates the brain.
Get involved in Social activities this will stimulate your brain apart from keeping you active and satisfying.
Learn a New Language.
This is probably the most stimulating of all mental exercises. It stimulates your recollection and memory.
As we grow older our memory begins to fade. Amnesia or forgetfulness begins to haunt us. Around age 60 our recollection becomes more difficult. Slowly & steadily we become totally forgetful or demented. This is age Alzheimer's disease (Dementia).
Learning a new language requires memory association to its fullest. It can delay forgetfulness or dementia well past hundred years of age.