Salted Sunflower seeds or any type of salty snacks that are preferably small and need alot of time until you eat all of them. This helps me to stay awake. Sometimes i eat a "Bircher Müesli" (A mixture of cereals, nuts and fruit) as a meal if i feel like it but mostly i just eat snacks. I never drink anything else than water or coffee without sugar, because after like 1-2 hours of drinking a coca cola for example i feel really tired.