Bananas are always a great idea. They're cheap, great for avoiding cramped muscles, and their wrapper is as biodegradable as it gets.
Popcorn or something else people can grab a fistful off and keep moving is also a wonderful idea.
Water is very needed, but we had people wavering at the June event due to lack of intake. Thankfully no one actually passed out, but a few got light-headed and had to sit down for a bit.
Anything you can provide in bulk will help out, really.