Classic. You can never go wrong with a good lemonade stand. Seeing as how this is normally a moneymaker for kids, assign your youngest to the task. With a little supervision and guidance, your kids can really have fun with this.
Though this may take a little bit of advanced planning and prep time, bake sales are great alongside of lemonade stands. This is also something that would be easy for children to run themselves once prices are established and baked goods are all packaged up and ready to go.
Though yard sales take a fair amount of advanced planning and labor, they are great because they kill two birds with one stone. They are wonderful moneymakers, and they free your house of the clutter that has been driving you mad for months!
Don't forget to check in with your city/township office to make sure a permit is not needed.