Tree/Play house heaven. This link goes to a website for a garden designer's portfolio. GORGEOUS pictures of many inspiring ideas for what can be done with outdoor spaces.
So much cooler than a tire swing and it won't collect water! <--- Have to disagree, nothing is cooler than a tire swing and if you drill a hole in the bottom they don't collect water.
slide into a hillside
ball pit playgroud
Great backyard tree house.
hobbit hole playhouse
recycle-old-tires palm trees
backyard zip line
dinosaur fossils at the bottom of the sandbox