High protein meals early in the day can stave off blood sugar spikes and crashes which makes a long day out in the sun much more bearable. I'm also really pleased that the festival brought back the water fill-up station. This is an important statement about water rights I'm very happy to see supported at Bluesfest. You can still buy bottled water at the festival (for $2, not as bad as it could be for them cash-hungry vendors) but it's a great option to have free water.
More on specific food recommendations later. So far, if you're GF (gluten-free) and generally love great food I recommend getting anything from the East India Company or a Greek Chicken Souvlaki Salad from Greek on Wheels. Otherwise you can get a full gluten-free pizza from Gabriel's Pizza (River Stage) but it's cooked from frozen and you need a crew to eat it with you.