You can still do the random decision thing - just a bit of pre-prep will make it easier. I usually decide what meat we're having each night in advance, then keep a selection of side dish staples in the cupboard/fridge. So I have salad stuff, veggies, pasta, rice etc all available, but generally won't decide how I am preparing said meat and what we're eating with it until the night I go to cook it.

For casseroles/pasta/stir fry type dishes, I make extra and freeze the leftovers. That way when I'm feeling entirely random, over it and can't be arsed I can just dig something out of the freezer and nuke it.