Swap a crib for the bottom bed on the IKEA Mydal bunk bed
6'x8' Covered Sandbox Plans | Not as super cute as the car design, but way cheaper. Note though, the cover is comprised of slats, and may not keep out leaves/pine needles.
DIY Sandbox with lid and benches
DIY Network, how to build covered sand box.
quick easy diy project no kreg jig required, $40 in materials
An awesome pallet bench! Add cushions, and you're set!
DIY - Old Fashioned Tire Swing for the gardens
I love this use of tires! It looks so basic and I bet it would get used in a lot of ways.
Wobble Step Bridge -- several cool ideas for an activity/agility trail from a company in Scotland
Backyard Playground Equipment - Open Travel