VERY IMPORTANT:
I am a caffeine addict, and I'm sure some of you may also be, so PLEASE learn from my mistake. I always got tired during the practice FL's, so for the real mcat I made a concoction of instant cofee with ice cubes, milk, and chocolate syrup. I drank this throughout test day for energy. I must have put about 5 cups worth of caffeine in it, but did not think about this while I kept sipping it (ok, chugging it) in between sections (I had put it in a water bottle). So of course it made me very alert during the test, but by the end of the BS section, I developed a blinding migraine. That's right, I could barely see the bubbles let alone fill out the right ones! I don't care about having a headache, but blurred vision and scantron bubbles don't mix!!! I ODed on caffeine on MCAT day!!! Please don't let this happen to you. Be careful!
ANyway, to answer the question, for breakfast I ate a bagel and a banana (good energy). For lunch I ate the usual-turkey sandwich, an apple, pretzels, and my coffee-concoction (BAD!). Don't eat anything that's hard to digest, you don't need your stomach making funny noises during the writing sample.
Another thing: BUY ERASABLE BLACK PENS!!! worked like a charm, although a little smudgy. It is a neat way to correct mistakes for the writing sample
Buy them now, a whole pack, and practice using them. They are good! Cross outs are ugly and you know it!