If you're not taking a digital camera, you might like to consider taking a mini camcorder. You can get some very good pocket-sized reviews these days. You could try the Panasonic SDR-S7 which records footage onto SD memory cards. Or maybe if you wanted to go smaller than that, try the Creative Vado for recording those unmissable moments with your mates. Read TechRadar's reviews of the Panasonic SDR-S7 and the Creative Vado
It'll drain your battery, but if this summer turns out to be a scorcher, you may be very grateful to have a portable fridge with you. It will enable you to bring some foodstuffs with you that would otherwise go off very quickly. Now, some people may consider this option to be sacrilege and not what festivals are supposed to be about. Still though, a car fridge is an option you should definitely consider. Try the Waeco Tropicool Bord-Bar 15L
It's such a basic item, but you wouldn't believe how many people go to festivals without a torch. Imagine fumbling around outside your tent, trying to get out of your muddy boots and into your sleeping bag without getting filth everywhere. LED torches are much more power efficient than traditional torches, and they give off a much more pleasant white-light. They're really cheap too. Check out the True Utility - TU80 - 21 LED Compact Truelite at Amazon.