I plan menus every week, including breakfast and lunch on Saturdays and Sundays. During the week, it's just me and the baby, and we eat leftovers or Ramen for lunch, plus oatmeal or cheese toast for breakfast. I actually cook those meals on the weekends.
I like to cook, which plays alot into it. I read cookbooks like novels, I subscribe to tons of magazines, and when something strikes me as interesting, I write it down for the next week's menu plan.
When I try to plan by the seat of my pants, we eat really, really simple and easy stuff, like refried bean burritos or quesadillas or grilled cheese and tomato soup. Tasty enough, but boring. So, by planning, I make sure that I get a chance to try all the recipes I stockpile. Otherwise, I'd get into a funk and think we never get to eat anything good, then we'd get take out to solve that, and it's a spiraling downhill kind of cycle.
So, anyway, to answer the question, I plan out the recipes I'm going to use, write down the page and cookbook right on the meal plan. I also plan sides, although my grocery list might say "something green to go with meatloaf." I wait until I'm there to see what looks good.