What other things should you do the morning of a big test to best prepare?
Protein is good. Depending on your dietary limitations (e.g., I don't know if you're vegetarian, keep kosher, have allergies or sensitivities, etc), this could be eggs, meat, Greek yogurt, or cheese, or some combination of these elements. If you don't want a big cooked breakfast, try Greek yogurt with a scoop of protein powder, maybe with some granola mixed in. Or if you like hot cereal, you can make "power" oatmeal or multi-grain hot cereal in the crockpot overnight, adding a scoop of vanilla or unflavored protein powder, plus some honey and/or dried or frozen fruit.
If you don't like breakfast food at all, have chicken or some other meat in a non-breakfasty form: dinner leftovers on a sandwich, a plate of rotisserie chicken (cold or warmed up), stir-fry.
A protein-based breakfast will fill you up so that you stay full longer, and it will give you longer-term energy than a carbohydrate breakfast. Donuts are tempting, but not a great choice, especially when you need clarity of mind. When you're nervous, you may not have much appetite, but do what you can to give yourself the best possible start.
*Get up early, so you're not rushed and stressed.
*If you have time, go for a short walk to clear your mind.
*Do some stretching and relaxing exercises to prepare yourself for sitting.
*Drink water. Being dehydrated doesn't help the mental processes.
*Do something to give your mind a break from the test: a crossword puzzle or Sudoku, ten minutes of reading a book you like. Something that's not numbing or stress-producing, like television or the internet, and something that's not at all related to the task ahead of you, will help your brain to hit "refresh" before the big moment.