Reminiscence therapy is one of the most popular psychosocial interventions in dementia care, and is highly rated by staff and participants. There is some evidence to suggest it is effective in improving mood in older people without dementia. Its effects on mood, cognition and well-being in dementia are less well understood
I find that my mother enjoys talking about and looking a very old pictures. I ask her to tell me things about the person and her memories. Not only does this put her in a good mood and frame of mind, it also serves as a way to "exercise her brain". You might try this and see how it works for you.