One clever idea is to use them as a mold to make beautiful cement planters. You can use the box as is to create a planter that's the same shape, or you can cut the cardboard into various shapes ...

Just as you can reuse them to create DIY flower pots, you can also turn them ... Placed right side up, they make for a fun mold. Once the cement itself has hardened, you'll use it to build a ...