Given delays, sometimes needing to run for a connection, etc., I usually keep myself fed, and that often requires eating on board. I'm sorry for my neighbors who may smell my food and have none for themselves, but that's not really my problem. Most any airport food place will sell you a carry-on version. I try to go for stuff that carries easily and is unlikely to make a mess or be awkward to eat. That often means a burger, cold sandwich, or a small pizza. I'm also partial to the Chinese food in the segmented trays though have to be a bit careful with it not to spill and I always grab a wad of extra napkins.

If I'm leaving from home, or have a fridge in my hotel, I sometimes take stuff like store-bought pizza, bagels+cheese, or finger food ("chicken fingers") and wrap it in my wool hat (which I always travel with) to keep cool. A piece of fruit is nice. Used to carry yogurt but can't get that through security any more.

Usually I have a few nuts or one sort or another of granola bar in my bag for real emergency food.