A recent NIH-funded study reportedly shows some cognitive function improvements in normal seniors from playing the video game "Double Decision," which was designed by a San Francisco-based software company to help stave off mental decay. I didn't actually read the study to confirm this, but the related papers I did read suggested this was basically "good science."
The effect of playing this game was apparently better than doing crossword puzzles on the tested cognitive functions.
What does the game do? Exercises your visual attentiveness, very-short-term memory, and your ability to focus. These are much the same skills as doing jigsaw puzzles, and having tried the game, I can tell you doing jigsaw puzzles is infinitely more fun.
You can try the game yourself (free, online) by clicking HERE, but warning! the instructions are very bad - here are some clearer instructions (once you click through to the START button):
1) First they show you 1 of 2 different cars, and a road sign in 1 of 6 possible locations. Memorize which car and where the road sign is.
2) Then the screen goes blank and they show you some black/white fuzz.
3) Then they show the 2 different cars, and you are supposed to click on the car you just memorized.
4) Then click on the location of the road sign you just memorized.
5) Then the "game" repeats.
I found it to be the mental equivalent of eating cardboard, but hey, see for yourself. Then go get some more jigsaw puzzles :) - not only do they exercise your visual attentiveness and short-term memory, but you get some fine-motor finger exercise as well, and you can play with friends! :).