Water, apple, handful of nuts, Walk around for at least 15 minutes. If a headache isn't on the way out then maybe try to shift the problem to your liver with some ibuprofen or Tylenol but it shouldn't be the first thing people try.
And just eat in general. My goodness sometimes I'm so surprised, I'll be feeling terrible (usual for me) and I'll realize I haven't eaten in a long time and take like one bite of something and instantly my headache and possibly fatigue are significantly better.