I have often made muffin batter and cookie dough up ahead of time and baked at a later time with much success. In fact, I prefer letting some batters to sit for a bit. My recipe for bran muffins (using all bran cereal) comes out *much* nicer when the cereal has had time to soften more.
This recipe is a large batter than you can keep refrigerated for several weeks (make only as many as you want). I often make a double or triple batch of chocolate chip cookies and bake only a single batch at a time. I use old 4 L plastic ice cream containers to store the batter / dough in the fridge (to keep it fresh).
I have not had any muffin or cookie flop because of baking them at a later time. Try it and you will probably be pleasantly surprised.