Actually another interesting exercise for working out the mind is learning a foreign language.
In Australia, you have quite a few people that can already speak another language, however for a few of us that only know English growing up, trying to learn another language is a great 'brain' exercise in itself.
My first attempt at this is studying Indonesian about 15 years ago as an additional subject at Uni.
Anyhow, doing it all over again now learning Italian (with a trip planned next year) ... some reading this will laugh, but to me it is an incredibly tough, learning a foreign language, with having male and female word changes for objects ... anyhow a great brain exercise in itself (also I believe the DS may have a French and Spanish program ... unfortunately no Italian at this stage).