On normal skirmishes, water and occasionally a packet of mentos or polo's for a bit of something tasty. Lunch times a sandwich, crisps and some biscuits but back at the car.
On longer games where I won't be coming in for lunch I carry things that I can pick at. High salt especially during the summer or when it is particularly hot. When its really hot and I know stopping will be a problem I will include isotonic drinks, last year during a milsim I got quite severely dehydrated and was on the brink of having to require medical intervention. Since then I take making sure I have enough salt and water quite seriously.
Tescos have these snack bags ( like half size sandwich bags). Three in my PC will contain, salted peanuts, biltong and dried fruit. I've recently been introduced to tuna in pouches, they look hideous but taste pretty good, especially the lime and pepper or tomato and herb ones from John West. Easy to eat in the field just tear off the corner and squeeze it out. Another easy to carry food I've been carrying round in pockets for quick snacks for quite a long time now are frubes, those yoghurt tubes you put in kids lunches. They make great easy snacks. Started that one 11 years ago at my first festival.
I try to avoid chocolate bars as they have a habit of melting.
My requirements of snacks are, small, lightweight (or atleast good nutrition for weight ratio), quiet to carry and eat.