Meatballs! We used to buy them, but after I started making our own at home, store-bought ones just don't do it for us anymore. And meatballs are probably the simplest of the make-ahead foods you can do at home - very little effort, resources and ingredients compared to, for example, ravioli. I usually make enough to eat in the evening I make them, and then a few servings for later use. Frozen, they can store in the fridge for a long time.