Advice when flying for long hours

I just did a 15 hour flight, with a stopover in Frankfurt. The total travel time with waiting at the airport and finally getting to our hostel was 21 hours. I only ate 2 clementines that whole time and I drank lots of water. I hardly slept either. I'm traveling with 3 others who ate, one mostly raw (my son), the other two(my Dad and nephew) ate the plane food. I'm the only one of the gang that felt great after the flight. I was able to walk for another 3 hours to sightsee and do some food shopping for the gang. Everyone else conked out and two even have a "cold"(my Dad and nephew). Eating lightly or not at all and keeping well hydrated are the secrets to a successful flight. The flight is already stressful enough without the body having to deal with food/frankenfood digestion as well.