Be the most awesome parent ever, and keep them occupied so you can relax (at least a little bit). Or let them help you build it, if you're feeling brave.
1. This cool tool bench used to be a thrift store nightstand.
You can customize the paint color too. From The Frugal Granny.
Source: thefrugalgranny.blogspot.com
2. A refrigerator box becomes a slide.
Only if the kids decide not to make a fort out of it. From The Contemplative Creative.
Source: thecontemplativecreative.blogspot.com
3. Old sheets and some wood make a campground.
These tents fold up for easy-ish storage too. From Lindsay and Drew.
Source: lindsayandandrew.blogspot.com