Hitchhike food consists of cookies, crackers, granola bars, etc.; anything that will give you energy, while at the same time being small, and not too prone to being smashed in your pack (such as potato chips).
If your budget is not so tight you will probably eat a lot at gas stations. Some of them can pleasantly surprise you, for example some gas stations in Finland that serve fries and self-service salad for only 2 euros. Avoid gas stations in France at all costs, unless you consider 3-4 euros a good deal for a bag of chips and a 33ml carton of orange juice. In Poland, fast food like hot dog can cost only about 3 PLN (1 EUR = 3.50 PLN).
Free food is plentiful in much of Europe. Try sandwich shops, bakeries, patisseries and other places like that around closing time. Often you get enough bocadillos to last for a few days!
On the back of receipts at all Burger King restaurants in the U.S., there is a toll-free phone number and a blank coupon. Call the number and you will be connected to an automated customer satisfaction poll. At the end of the poll, you will be given a code to enter into the blank spot on the coupon on the back of your receipt. The coupon is good for a free chicken sandwich or small whopper with purchase of a drink or fries. At some Burger King's, your meal is free if they don't give you a receipt (there's no way you can lose!).