Definitely reading (newspapers, novels, non-fiction...pretty much anything). Try playing word or card games. You can go to http://funadvice.com/r/3ju9kotlsr for a ton of games on the internet (you're able to chat with people if you want, and play tons of different games). Some say that ginko biloba helps with memory etc (I don't think it's been proven, but you could talk to your MD about it).
sudoku, brain training games? crossword, spelling everyday objects backwards in your head. making your own puzzles playing snap +24 (i think that is the name of the game) when you have a card with lots on numbers on and you can only use them once to get to the number 24 using multiplication, division etc
Find some little kids to baby-sit. They might drive you crazy but they will keep you sharp! What hobbies do you enjoy? Check online for clubs in your area that you could join, such as people who play Scrabble or Chess, a writer's group or poetry readings.
Maybe bingo, try walkin around the block to stay active
Crossword puzzles,word finds,reading the dictonary.
read the original sherlock holmes and do sudoku!
thank you everyone for your advice
Study a foreign language.