Play strategy games, or try games specifically designed to exercise your brain.
Read and write daily. Especially with non-dominant hand if possible.
Seek new activities and unfamiliar settings, like traveling or a new hobby.
Continue your education by taking a class or learning a new language.
Try doing things with your non-dominant hand. If you’re right-handed, use your left hand to brush your teeth and open doors, or take the first stair with your left leg.
Take up new hobbies. Hobbies involving hand-eye coordination and mental calculation (like sewing, wood-working) are great ways to involve different parts of the brain. Either playing or listening to music can have a positive effect on the brain. (Alzheimers Info)