Learning to play a musical instrument is highly recommended for mental stimulation. If your fingers aren’t strong enough or fast enough for that, learning a foreign language is another option. I have a number of Rosetta Stone course on CD, or you can download them. There are language apps if you have a tablet, but I haven’t found one that compared well to Rosetta Stone.
Learning some kind of craft is also a good idea. The choice of craft will depend on where your interests lie and on your manual dexterity and strength, and also on your eyesight. I like to knit “continental style” - your hands move less with this method and there’s reduced risk of developing RSI. I used to embroider, but my eyesight and manual dexterity aren’t what they were. Painting and drawing sound good, too. I’ve always wanted to be able to do both, but I’ve never got round to it.
Word puzzles of any kind are good for keeping you mentally active, as is sudoku. I always enjoyed logic puzzles. I have a lot of jigsaw puzzles that I’ve been meaning to do for a long time. Maybe this Christmas, if I’m not knitting.
One final suggestion: writing. You could try poetry, if that’s what you like, or short stories, or something for children. It doesn’t have to be great literature, and it doesn’t matter if you don’t finish the story. Just use your imagination, give your brain a different kind of exercise.