I usually had something from the local backery on the way through, usually some cheese and bacon rolls and a choc milk... except for the business studies exam, I rocked up to that in a taxi still absolutely trashed from the night before and was afraid anything I ate would be seen in half an hours time.
Just eat what your used to eating, don't try and change it for one day because it won't do anything drastic, just go for the usual. Just make sure its substantial because you can't concentrate if your hungry.