Snacks are extremely important when out hiking or during prolonged exercise. To avoid "hitting the wall", a snack is recommended every 60-90 minutes for backpackers or during sustained physical efforts. A snack should be picked carefully to provide the right nutrition. Try loading up on hiking snacks that boast a balance of carbohydrates, fats and protein.
Nutrition isn't the only factor to consider. Weight and portability are also important. Try to choose light weight, compact and shelf stable snacks. The temperature should also be considered, as during hot months, some snacks (such as chocolate) may melt or soften, while they may be perfectly suitable during cooler weather.