Avoid stuff like Carnation instant breakfast...these have little protein and a lot of simple sugar carbs.
Breakfast is important. You've been fasting for about 8-10 hours while asleep, so the body wants quality food.
A breakfast with complex carbs, fiber, and protein is ideal. it'll refuel you, and stick with you all morning.
I tend to have eggs and oatmeal almost every day. more specificaly, 5 egg whites and 1 or 2 egg yolks, which gives me 20-23g of protein.
1/2 cup (uncooked) Old fashioned Quaker oatmeal, takes about 2 mins to microwave. around 30g of carbs, and several grams of fiber. i put splenda, cinnamon and sometimes a tablespoon of natty peanut butter in it.
for my second breakfast, around 10-10:30am, i'll knock back a protein shake (Optimum nutrition, pro complex, chocolate flavor) and then its either a pack of instant oatmeal, or some other kind of complex carb, and maybe some fruit.
I'm a very strong advocate of eating 5-6, sometimes 7, meals a day. it really keeps your metabolism going, and you'll have energy all day. Since I don't gorge myself during lunch, especially not eating fast food, I'm not lethargic all afternoon like my co-workers.