In my experience, this is the ultimate travel food: grainy bread and a ripe avocado. It's simple, healthy, comforting, filling, and easily portable (and not a liquid, so you can get it through security). It also doesn't have much of an odor, so your fellow passengers aren't disturbed (sat next to a guy eating a breakfast biscuit on my most recent flight home which was unpleasant to say the least). Grab a disposable plastic knife somewhere on your travels (or ask your friendly flight attendant) and you're good to go. I cut my avocado in half and then use each half as a little bowl to chop up the contents before spreading them on a chunk of bread, but you can enjoy these two paired any which way you like!
I picked up this Dave's Killer Bread Seed Bomb at Food Fight in Portland, OR and the avocado (requesting "one that will be good to eat tomorrow, please!") at a produce stand in Pike Place Market in Seattle (the produce and flower markets there are awesome, but the stinky, gross fish markets kind of ruin the fun).