I always make myself some "snack bags" with different nuts (walnuts, pecans, etc..) because they don't even give you a nut choice now when flying it's either cookies or some kind of cracker...and I don't want either of those options...

If the day is going to be long, I will also buy a small summer sausage (I like the smoked ones) and slice it up and put it in a ziplock to take, it'll keep just fine for a day and usually does the trick for me. I also take the time to try to "push" my fast window a little (normally eat at noon each day til 8 pm...sometimes I'll try to push that to 2 or 3pm (usually whatever the last leg of my trip is) and then eat a healthy portion of the meat/nuts to hold me over until I can get into a hotel and get to a grocery store...