I've got diabetes, so eat very low carbs, as the alternative would be to be on insulin. So, no bread or other carby foods.
So for a day trip, a good breakfast of steak, chops, sausages bacon and eggs, in some combination or other before heading out, and snacks of pepperoni, pickled octopus, tinned oysters or mussels and stuff like that. If organised, I'll pack a Greek salad for lunch. Drinks include tea, coffee, soda water and sugar free energy drinks, particularly the day after drinking rum and or red wine.
For overnight trips, pretty much the same, with something to put on the grill for dinner with salad, or some coleslaw if I am lazy. Cutlets are good for dinner or breakfast, but getting to be a luxury item these days. A BBQ chook is always worth chucking into the cooler, too.