This idea alone has saved me more money than almost anything else. I will try and give a few examples.
I pack a large cooler before we head out. I also pack a large bag of non-perishable items for convenience sake. I first and foremost bring lots of bottled water. You can get a 24 pack for nearly the price of a single bottle at other tourist locations. I bring bread, peanut butter and jelly, cereal, milk, granola bars, beef sticks, value pack of course, cheese, fruit cups, dried fruit, nuts, trail mix, crackers, cookies, etc. We plan ahead of time to eat out only once on our vacation. It makes it so much more exciting and special! No one complains and our food costs are shockingly low, bringing the cost of the trips down immensely.
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Take a thermos with you on family outings to avoid eating in expensive restaurants. You can fill them with hot dogs, sloppy Joe, BBQ beef, tuna salad, etc.