What to pack a healthy lunch (can't warm )

Raw veggies - carrots, celery, peppers etc - cut into chunky batons and in little plastic bag. If you can get hold of the little, disposable, plastic pots with lids, fill a couple with hummus or an avocado dip. You can actually freeze the pots, and just take them out before you leave home. They will, of course, be defrosted by lunchtime, but it's a useful way to do things, if you don't want to haul around relatively heavy ice-packs in your lunch bag. I'd also put a load of grapes in another plastic bag - you can even freeze them, to ensure they're at least a little cold! Then add in stuff that is easily transportable - small bag of unflavoured pita chips, low-sugar granola bar, little bag of unsalted nuts etc - that are also easy grab-and-go snacks.

Finally, it's always worth checking out the festival website, in advance. They'll often say which food vendors are attending. You'd be surprised just how many festivals these days have a real profusion of 'real food' type small suppliers - often with pretty healthy options. The last two festivals I attended were fantastic for food choices - and one food truck in particular, served amazing vegan Indian food, and had incredibly filling, nutritious meals, all falling under that 500 cal limit. The lentil curry they had was one of my favourite Indian foods I've ever eaten! At the very least, you can work out which vendors are absolute no-nos, and make very wide circles around them to avoid their delicious aromas of cooking!

Subway sanwich?

You can make homemade granola bars and stuff them in a bag - it should be easy to sneak it in depending on how you pack.