Reading books, solving math problems, learning languages, programming, learning to play musical instruments.
No vitamins, dietary supplements, nootropics and other "magic pills" will help you. Forgive me if I snatched from the world of illusions into harsh reality.
On tjournal recently there was a detailed article about this kind of brain stimulants. (tjournal.ru)
It is worth mentioning that scientists are very distrustful of this whole thing. Like, even if there are no obvious side effects, it is still better not to use these drugs on an ongoing basis.