You're generally better off eating as healthy as possible the days leading up to the test and the morning of. Avoid heavily processed foods like frozen dinners and pizza (they'll sap your energy and ability to concentrate), and eat more protein for brain power. Don't make any last-minute changes to your diet that are significant, because your body can react badly, esp. with the extra stress of the test.
When I took my first GMAT, I was sick and feeling really lousy, so I ate a high-protein breakfast. I overdid it and got sick on every break! Needless to say there's a reason I call it my "first" GMAT!!
Go easy on heavy carbs, esp on test day, because they can slow you down as you digest them. Apples are great but bananas take longer to digest and can bring you down in the meantime. Avoid processed sugars as well, although if you need a boost during a break, a Snickers bar or some peanut M&Ms will give you both a little sugar boost and a protein boost from the peanuts. Don't do too much sugar, because when you're stressed it can more quickly and strongly affect your blood sugar levels, and if you crash during the test, you'll be hating life.
I'd bring a protein bar, nuts, an apple, water, and something like Cheerios with you for the breaks.
And one more rambly thought -- if you don't normally drink coffee, don't drink it the day of the test. If you normally do, then drink it. Go with what your body's used to.
Good luck!