This rainbow of DS Lite styli can be yours for the low, low price of a few minutes on Nintendo's website. Brain Age 2
owners can register on my.nintendo.com (which we should all do anyway for the extremely sporadic bonuses like this) and take a short survey about how awesome the game is, and in return they'll get a "gift box" containing these seven styli!
Unfortunately, while our brains may be well-trained, our browsers may not all be in such great condition: some would-be applicants are reporting technical difficulties with the site-- specifically that the survey doesn't pop up. Maybe it's a training exercise for Nintendo's IT team.