When my kids were growing up and on swim team, they got used to combinations of: fresh fruit (banana, apple, grapes, pear...) chunk of bread torn off loaf (usually whole grain), chunk of cheese broken from block, raw veggetable (which if you were lucky, mom not only washed and cut up, but also peeled if needed and paired with something to dip it in - like a cup of yogurt or some nut butter), drink (water bottle, can of juice, shelf stable milk that may or may not have thawed...) maybe nuts or pretzels.
they complained. They were hungry. They ate and lived to complain again.