Gas station or rest stop convenience stations:

string cheese or sometimes they will have babybel or even fresh cut cubes, if they have "deli"

deli meat!- i love when they have a boars head deli and you can order a sandwich and eat guts. one time i ordered triple meat, as traffic was 12 hour drive instead of 4 due to overturned tractor trailer, and my dog was hungry. i got quite a shocked look on that order

sometimes they have a great big individually wrapped pickle

hardboiled egg

nuts or seeds

jerky- my favorite right now is buffalo chicken!

a cup of coffee- you can mix in a scoop of protein powder (chocolate) that you keep in glove box or wherever with emergency baggy of one scoop per bag. stir in for a "poor man's mocha"

salads and little containers f celery/carrots with ranch are also often available

few other chains that are easy to customize:
chipotle or moe's burritos- both have great nutrition calculators, and you can get your stuff served in a bowl on lettuce- so think fajita chicken or steak, fresh salsa, sour cream, cheese, guac.....

uno's
has loads of pub style stuff like burgers, grilled chicken sandwiches, wings, salads with nice meat, etc....artichoke dip, etc, etc.....huge menu.