Buy a cooler or insulated backpack or anything you could carry with you. There are tons of non perishable items you could carry with.
Jerky, nuts, nut butters, seeds, fruit, good bread, jams, cheese, dried fruit, veggies. Hell there are plenty of dishes you can cook up and bring in tupperware. Its just laziness if your only choices are nothing or doritos. I added bread in there because i think it would be worlds healthier to eat your own homemade sandwhiches then a storebought one.