I send fruit to my son's preschool for snack a lot. Apples, peaches, pears, etc make a good snack sliced up. I just put them in a big container and they pass it out to the kids. I usually plan for each kid to get 4 or 5 slices. Grapes, blueberries, chunks of melon, cherry tomatoes, all work well too and you can just do the same thing. If they don't have stuff to serve them in you can just get a pack of small cups for the applesauce.
They also love cheese and crackers and sometimes I make muffins to send in (tonight I'm making whole wheat butternut squash muffins with cinnamon for tomorrow's snack). I have to make for ~40 kids, so I don't do it that often because it's a lot of work in the evening.
Another easy one that the kids love is cereal. Buy a big box of your son's favorite, a gallon of milk and some cups to eat it out of and they love it.