+1 on learning a new language! I've spent extensive time over the past couple of years learning Spanish, and living in Mexico for a year gave my brain a real workout. However, I can tell that learning Spanish has been harder for me than French was back in high school/college. I just don't have as good a memory as I did back then, though I still understand the concepts, of course.

I do lots of reading and although some of it is easy mysteries and such, I do try to challenge myself by reading historical fiction and other genres that are a bit more challenging. I'm currently participating in a 2016 "Read Harder" challenge that has had me reading things like science books, books by authors of different ethnicities, etc. It's been fascinating.

Since I'm no longer working, I have to try twice as hard to keep the old noggin active. Fortunately, traveling full-time as we do and planning for these travels, is also a great brain exercise; dealing with maps, finding our way around new places, and visiting historic sites, etc. Even cycling and figuring out routes has got to be as good a brain exercise as crossword puzzles, I think!