Sodas: Regular sodas are loaded with sugar, which gives you an immediate blood sugar spike. This translates to a quick energy burst with a huge crash shortly afterward. Sadly, any caffeine kick will have worn off by then as well.
Large sandwiches: A large serving of bread, processed meat, and greasy condiments aren't a recipe for a healthy lunch. The carbs in the loaf of bread will give you a carb crash, and lunch meat won't do much to boost your energy.
Oatmeal: It's a great energizer but a lot of oatmeal on the market contains as many carbohydrates as a bagel. And the sugar people add to sweeten their oatmeal up can cause an eventual afternoon comedown.
Cereal: It's not a big secret that a lot of breakfast cereals contain a high amount of sugary carbohydrates. Sugar releases insulin in your body, and insulin in turn releases the amino acid tryptophan in the brain, which then makes you feel tired.