I try to keep a stick of pepperoni (un-opened), some macs, and a bottle of water in the car, but just like everything else someone may decide that they want to eat my stash and not replace it. So on those days.....

Convenience store (not the best option)

Nuts
meat sticks w/cheese
sliced lunch meat (if they have a deli)
pork rinds

Fast food (all time fav.)

Wendy's triple beef w/ cheese pickle onion and bacon no bun.
This will keep me full for the rest of the day while I run my errands.