Fruit
I know this seems like a stock answer, but fruit really is a perfect energy-boosting snack. Apples are full of vitamins and antioxidants and bananas are packed with potassium. If for some reason, you don't like eating fruit by the piece, that's okay. Fruit smoothies are just as good, and you can get many different kinds of fruit at the same time.
Cucumbers
These are a great source of nutrients. In addition, they are great for hydration and full of silicon, a mineral that helps keep your hair healthy. Your first instinct may be to dip them in ranch dressing or sour cream, but you might also consider vinegar or hummus.
Eggs
Eggs are great for energy, as they are full of protein. They only have 80 calories, and they are full of other nutrients. You may want to avoid the yolk if you are worried about cholesterol, but the yolk is full choline, a memory helping nutrient.
Water
This may seem like a strange choice, but keeping hydrated is a good way to maintain your energy. Your body depends on all of your nutritional needs to keep its energy up, and the primary bodily need is water. If you don't like drinking plain water, you might also try fruit juice or lemon water.
If you liked these, there is another great list of energy foods here.
