You can build monkey bars in your backyard in a weekend for around $100, AND they will be bigger and better than sets you can buy for thousands!
Reclaimed fence posts recycled into a balance beam for kids garden play area.
Not loving the mustard yellow, but a blue on the cement slab in my back yard would be perfect.
Paint the tops of tree stumps with glow in the dark paint. Fun to climb on night or day! (via wohnideen.minimalisti.com) Site is in German