Before having kids, I despised dealing with the cardboard boxes that came with every UPS delivery. Now that I'm a parent, good cardboard is as good as gold.
One of our favorites we've seen is this colorful castle. The architects? Christiane Lemieux's kids. (Yes, THAT Christiane, the founder of DwellStudio). We love how the castle interconnects, but our favorite has to be the colored masking tape. The children really used it to great effect. For more photos and tips on how to create one of your own, check out Built By Kids.
(Image: Christaine Lemieux via Built By Kids)