i always have some sort of snacks in my bag. granola bars, nuts, applesauce, raisins, peanut butter or cheese crackers, fruit cups, tuna salad cracker kits, or anything shelf stable and portable that i can toss in there til i find myself off schedule. i try to keep at least three different kinds of things in there, and i make sure to replenish when i dip in.
i also have a list of some of my favorites at the local grocery store: fresh sushi, premade salad or fruit, yogurt, hardboiled eggs, sometimes soups, and anything that would make my nonperishable bag snack list.
then i have a third tier list of fast food places i would stop, and the decent options i prefer there.
i think that one of the most important things to plan is what you do in the off situations. it's great if you can set down a meal plan a week out in advance and stick to it [i've never been able to, but it seems to work for so many people], but most people find themselves in a press of time or otherwise off of where they thought they would be, so having a plan in place for that [particularly if you have blood sugar issues] is going to be a key point. and if you find that something like taco bell doesn't work in there, you evaluate. was there something else that you could have gotten at taco bell they wouldn't have sent you spiraling? what's near taco bell? the closest one to me has a mcdonald's, a popeye's, a burrito place, and a publix within rock throwing distance and a cuban deli, a bagel place, a deli and one or two more that i am forgetting within about a block or two of it. what other places are close by and have better options if you need to divert?
-google first. ask questions later.