The most common way to bring lunch on a field trip as a chaperone is to pack a bagged lunch, just like the students. However, while students may be content with peanut butter and jelly and a bag of chips, you may want something healthier and more sustaining. Consider a sandwich wrap with lots of vegetables, and bring condiments on the side to keep it from becoming soggy. A container of hummus and pita bread is a lunch that travels well, is healthy and filling. A bag of baby carrots, cheese wrapped in foil, and any kind of crackers are also good options. Avoid fruits, as they may rot in the bag. Also, avoid anything that may need to be heated or prepared, as you may not know where you will end up eating the lunch.
Depending on the nature of your trip, it may make sense to budget a lunch out at a restaurant rather than asking everyone to bring lunches. As a chaperone, you may not want to eat lunch at McDonald's, but the students on the trip may not want to go to the restaurant of your choosing. The best way to deal with lunch out is to find a place with multiple options in close proximity to each other, such as a town center, a food court or a supermarket. If you do not want to let students go on their own to seek out lunch, choose a few options and assign a chaperone to escort students to each.
One good strategy in bringing lunch on a field trip is to combine forces with the other chaperones. Like a pot luck lunch, everyone can bring a component to a meal (salad, cheese and crackers, bread, dessert). This way, you can enjoy a full meal without having to worry about preparing and transporting more than one container of food.