I spend a lot of time at all day regattas and always bring way more than I ever really need. So my suggestion is to pack light. Take a sandwich that can survive the heat for a mid-day meal (I almost always pack a falafel since there’s no meat or dairy to spoil). A couple pieces of fruit. And maybe a small thing of trail mix (I like the kind with M&M’s mixed in) or small bag of Fig Newtons. That’s really all you’ll need. Otherwise you’ll have a whole lot of icky, mushed up food left over at the end of the day that you’ll just toss.
Funny timing, tremorviolet, because that’s the conclusion I came to yesterday while I was at the grocery store. 
I had all sorts of stuff on my list, but then I realized that it’s not like I’m going out into the wilderness for a week: I’ll be in the middle of a large outdoor shopping center, surrounded by Starbucks and restaurants and whatever food is being sold at the festival. I just don’t want to rely on the sugary, vending machine snacks that the volunteers get for free (and that, last year, were yucky when hot), but I couldn’t think of any good ideas before I posted here. Now, I pretty much have it down to just a bag of banana chips – I’m hoping that they won’t stick together when it gets hot. I might throw some Fig Newtons in there, in case I start jonesing for something that doesn’t crunch, but I think I’ll be ok packing very light.
I got some excellent ideas from everyone here … and if I ever decide to start making my own jerky, I’ll know who to ask for help! 