It sounds like any good exercise will do.
From Sunday's NYTimes:
For more than a decade, neuroscientists and physiologists have been gathering evidence of the beneficial relationship between exercise and brainpower. But the newest findings make it clear that this isn’t just a relationship; it is the relationship. Using sophisticated technologies to examine the workings of individual neurons — and the makeup of brain matter itself — scientists in just the past few months have discovered that exercise appears to build a brain that resists physical shrinkage and enhance cognitive flexibility. Exercise, the latest neuroscience suggests, does more to bolster thinking than thinking does .(emphasis added.)
Of course every week there is some new food or activity to add or avoid. But walking and running have been on the good list a long time now. None of these mean you (specfically) will or won't live longer and get smarter, but it means people in general will, and you might be in the group that does.
[UPDATE: See follow up post: It Worked, I Ran I Got Smarter. ]