I always plan the menus a week (or more) in advance. It's easy for us, because there's little variability in breakfast and lunch, and currently our evening meals are on a 1-2 week rotation, generally speaking. It's easier that way with little ones - we are working on expanding their palates, but we do have a couple of things that are always on the weekly menu.
Anyway, I like the idea of cooking from what's on hand and only buying produce loss leaders and bulk staples, but in practice, I wind up with a few items that are in the grocery cart no matter what - bananas, cherry tomatoes, carrots - because my kids go ape for them and we are guaranteed regular fruit and veggie consumption for them.
For me, breakfast is always oatmeal, frozen berries, banana, protein powder. For my wife, eggs and bacon. My kids eat cereal and fruit.
Lunch is sandwiches for the kids, legumes, veggies, and grains for me, and leftovers for my wife.
Dinner is more variable, but as I said, we have some standard items, like weekly spaghetti and our Friday night "tapas" (mostly cheese and crackers for the kids, but this meal is a good way to get them to try a bite or two of the "weird" things Mom and Dad are eating with their cheese - olives, fried artichokes, rice balls, etc.)
Grocery shopping is easy and efficient this way, and I can get in and out on Friday mornings before work most weeks, even using the bike trailer.