Since I've retired and don't interact with many people on a daily basis, I was afraid that I would "stagnate", for lack of a better work, without more daily mental stimulation. I have fallen into a few habits which I feel keep my brain percolating. They also happen to be fun.
The first is a computer Scrabble game, which I have always loved to play. It's pretty hard, even at the lower levels because, well, the computer has the whole dictionary in its memory, and I definitely can't say the same. Nonetheless, it's a great tool to keep the vocabulary skills up. I play a few games every day.
The second is Sudoku, which I didn't really like at first but has become addictive. I have several puzzle books and pencils (with erasers!) lying around the house for odd moments, and I work a few before I turn in at night. Good for keeping the logic circuits lubricated and running smoothly.
Add in my quilting for stimulating creativity, and I think I have a well-rounded brain program!