Have something like porridge or oat based stuff for breakfast with some protein.
I have yoghurt and banana at morning break.
At lunch either a vegetable based soup with a sandwich or oat cakes with something protein (sometimes smoked salmon or boiled egg) and then fruit. Or I have couscous (with harrissa mixed in) with tuna mayo/sweetcorn/cucumber/spring onion with then fruit. Or bread, salad and tinned sardines with then fruit.
Occassionally I have jacket potato with tuna from local shop but they are so huge I don't need anything else.
Afternoon snack tends to be whether we have time but I would take something like a nutty flapjack or a boiled egg or banana with oatcakes.
Also make sure you always have a glass of water to keep hydrated as dehydration causes tiredness and lack of concentration.