I used to go to science fiction conventions and the Hospitality Suite (aka ConSuite) would have plenty of snacks and even some "real food". My DD also attends swim meets and coaches and officials need to be fed. Here are some ideas.
* Prepare veggie plates in advance. Cut up carrots, celery, etc plus some ranch dip.
* Buy big boxes of snack food like pretzels and chips. I know our big box hardware store (Menards) has that kind of thing.
* Let people make their own sandwiches. Buy a couple of big jars of peanut butter and jelly, buy bread from a bakery thrift store. Buy sliced meat and cheese from Costco/BJs. Aldi's might be a good place for the PB&J and even bread.
* Buy bagels and cream cheese.
* Individual yogurts.
* Cold cereal & milk.
* Get a 6' sub from Subway. They can make is with a variety of toppings.
* Make chili & soup in advance, freeze it in a ziplock bag and defrost and warm in a crock pot. Make pulled pork the same way.
* Order in pizza. See if the pizza place will give you a deal.
* Grill hot dogs in advance, freeze them and cook them in a microwave (my friend does this for the winter so he can have "grilled" hot dogs when it's too cold to grill), serve with potato salad & chips. Or ... skip the grill and warm the hot dogs in a Nesco or crock pot (?) filled with hot water.
* Make a "baked potato bar". Cook big baked potatoes in advance and warm in microwave. Serve with sour cream, cheese, bacon bits, left over chili from before, salsa, etc.
* Make a "taco bar" with the same kind of stuff.
* Have a cooler of soda & water available at all times. Volunteers get really thirsty!