You can always make a cup-o-noodles with a little hot water from the galley. Throw in a couple sandwiches from home and eat some soup in ops when you get the longer breaks. I bring some peanut butter from hotel b-fast and make little cookie-peanut butter sandwiches. Sometimes it breaks the bordom of the day to figure out how to eat for free, why would you want to ruin that with crew meals?
My fav is get some ketchup packets, add hot water and there ya go, Tomato soup!
I can make most 4 days without spending any money of food, now beer, thats a different story.