I frequently take day trips during nice weather. Here's what I do.
If I have immediate access to my car, I use a cooler with ice packs (or frozen bottles of water) to keep my food cold.
If I have to carry my food, I keep a small insulated lunch bag with a small ice pack in my backpack. You can find these insulated lunch bags at most stores, especially during the late summer in the "Back to school" section.
Miscellaneous containers:
Plastic containers with tight fitting lids---these come in handy for salads, sandwiches, etc.
Plastic sandwich bags: Keep a couple in your backpack, and use as a "trashbag"
Thermos jar: for hot foods, like soup, beverages
Utensils
Plastic spoons, forks, knives
Camping utensil kit: These are a set of metal spoon, fork and knife. I found them for under $5.00 in the camping section at Walmart.
Collapsible cup: Optional item...
Hygiene
Paper or cloth napkins: I use cloth napkins because they come in handy to wrap things or to use as a quick "tablecloth" on questionably clean public picnic tables. They are also rather larger and washable.
Liquid hand sanitizer or disposable sanitizing cloths:
Foods: Sandwiches: Use a bread substitute that's legal for your phase of Atkins. Induction folks can use Atkins Revolution Rolls. Fill them with your usual sandwich fillings.
Tuna or chicken in foil packets: Bring along some packets of mayo for a quick tuna or chicken salad.
Eggs: unpeeled, hard-boiled eggs, wrapped and kept in cooler.
Cheese: be sure to keep cheese cool.
Veggies: cut veggies, like celery sticks, jicama sticks, etc. can be packed into sandwich bags or into smaller plastic containers with covers. Salads can be stored in a plastic container with a tight fitting lid.
Dips/Dressings: Find a small plastic container with a tight fitting lid. Fill it with the dip or dressing and freeze it overnight. Put the frozen dip/dressing container with the veggies. As the dip/dressing defrosts it will keep your veggies cool. Fruit: Either keep the fresh fruit whole or cut them and put in plastic containers with a tight fitting lid. Freeze-dried fruits are also available in some health food stores or on-line.
Nuts: store in a plastic sandwich bag Trail Snacks
Store in a small container or a plastic sandwich bag
Pepperoni chips
Cheese crisps
Flax wafers
Cut veggies
Beef or turkey jerky---read the labels to make sure they are low carb
Nuts and seeds
Freeze dried fruits Beverages: Water: freeze the bottles overnight.