1. Portable stove
In most cases, a portable stove is an approved item to bring to the field. One of the most common stoves is the convenient Jetboil. Available in a variety of sizes and powered by easily-packed fuel canisters, Jetboil stoves can be used to boil water to warm MREs, cook instant ramen, make coffee, and even shave with (just be sure to boil more water and wash it out afterwards). Also, be careful operating it so that you don't end up like Cpl. Ray Person after Rudy's espresso machine went off like a 40 mike mike (if you know, you know).
2. Folding stool
Look, laying in the prone for hours sucks, especially if the ground is cold, wet, and full of bugs. Some grunts and snake eaters live for that sort of thing, but for those of us that don't, there's the folding stool. When you come off the line and into the center of the patrol base for some chow, a stool to sit on is a simple luxury that can make all the difference. Plus, you won't have to fight with your buddies over who gets to sit on the few remaining unopened MRE boxes. You didn't hear it from me, but you might even consider popping a squat on your folding stool while pulling security. Just don't get caught.
3. Baby wipes
This one is almost a given, but you'd be surprised how often troops forget their baby wipes. If you do forget them, you'd better hope your buddies are willing to share theirs. Otherwise, it's the MRE napkins for you. Aside from helping you answer nature's call, baby wipes also provide the cooling relief of a "field shower", which can be all you need to unwind after a day of patrols. If you'll be wearing face camo during your time in the field, consider packing makeup wipes as well. They're better suited for cutting through the thick and waxy face paint than regular baby wipes.
4. Civilian food
Again, this one is almost a given. That said, packing lists can sometimes explicitly forbid bringing any type of food, so read yours carefully…or just hide your food well. Sunflower seeds can help relieve boredom when you're static for hours on end, beef jerky can keep you going during those long foot patrols, and a can of Chef Boyardee (heated with your portable stove) might be the morale boost you need to make it through the final days of your op. Need a convenient way to carry your food while having it easily accessible? Consider adding a magazine dump pouch to your kit. Just don't let First Sergeant catch you pulling gummy worms out of it.
5. Nintendo Gameboy
Or really, whatever portable game system you want. I've seen everything from PSPs in sunglasses cases to a Switch stuffed in a Crown Royal bag. That said, an old-school Gameboy offers you an easily concealable platform, absurdly long battery life, and the chance to beat the Elite Four again. A Gameboy can also be quickly switched off and shoved into your pocket or an empty mag pouch if you see your squad leader coming towards your position.
By no means is this an all-encompassing list. Really, the best items to bring with you on a field exercise are the things that will make you happy, whether that be a book, a photograph, or fuzzy socks to sleep in. Again, and I can't stress this enough, pay attention to your packing list…and your surroundings.