Vitamins ALWAYS, esp. C, D, E and K.
Plenty of water.
carbohydrates
and Omega-rich foods
so basically;
Fruits, veggies, fish (esp. salmon), nuts like almonds and flaxseed, DARK chocolate and bread!!
Alittle bit of empty calories is also FINE!!
just don't over do it
Lol, my mum makes me drink an omega 3 fish oil tablet a day. And some other random multi-vitamin. Though that may be more to do with my [lack of] height + weight than with stimulating brain activity.
Eat, eat, eat dammit... don't starve yourself (it never works and plus I hate feeling hungry)
Also do sport and you'll still be healthy and smart at the same time