I LOVE baking with my little girls. It doesn't matter how simple it is, they get excited to help.
Today, we did something simple. We mixed a spice cake mix with one can of club soda (or soda water) and then baked as usual. It was easy and they loved watching the bubbles and stirring it together.
For a fall theme, after the muffins were done the girls put leaf decals I got at the dollar store and they were totally excited.