The beauty of meal planning, and grocery shopping off of your plan, is the ability to move your meals around if you're "not in the mood" for something. As long as the food is in your pantry, fridge, and freezer- you can hop around all you like.
The only thing I make sure of, personally, is that I'm not wasting food by forgetting to use it until it's gone too bad to be used. Make sure your incorporate those fresh foods that won't keep- for example, instead of a stirfry, how about a vege plate on the side? Wasted food is wasted money, and one of the benefits of meal planning is saving money!
I'm not feeling great today. My meal plan suggested Thai pork wraps. I opted for a thicker pork and potato soup, instead. It's warm outside, but I'm tired, so I'm cold. And soup is always comforting. The wraps will be good for another few days. I used up our last 3 potatoes before they got too old. There's more pork in the freezer, and the leftover carrots from the other night got used too.
We can always have pork wraps Friday night, instead.
How do you improvise your meals?