Let the kids go crazy with a Yoodo!
The Yoodo by Offi is a modular system made of extra-heavy weight cardboad designed for kids to build a fort or otherwise secret playhouse... just throw a sheet over the top? Remember that?! The Yoodo construction creates fun places for the kids to crawl through, create windows (puppet show?) or just makes some off the all race car (that perhaps would be the case in our house....).
The Yoodo kit includes 16 big + little pieces and is currently on sale for $29. And while the red + white is nice, I'd be tempted to let the kids loose with a few markers and some stickers to really sweeten it up.