Fruit is always good cos it gives you a sugar lift, but not a nasty unnatural one. A banana always does the trick with me, 'cos it's more substantial than an apple, so you kind of wake up, because you are having to digest food, but it also gives you an energy boost.
Also foods that release their energy slowly and at a constant rate, porridge oats are a great one! Also avoid caffeine, which may give you an instant boost, but one that soon wears off!