Besides removing large bushes the kids could hide behind, we created two play areas, though here my wife and I went in opposite directions. While her style is more towards structure and organization, my style is more towards deconstruction and imagination. So her project involved lots of lumber, bolts, brackets plastic pieces and gravel. Mine involved dirt and water.
Of course I have the advantage of being a boy (still), and have a long and rich lifetime of memories to draw upon. From pulling apart a neighbor's brick patio and creating a series of canals across the entire back yard, to scavenging large pieces of styrofoam and going floating out in the San Francisco bay (much to the horror of the school field trip supervisors); I just know that water and dirt are a hard combination to beat.
Heck, even just dirt is pretty irresistible, but when you add something to splash and make mud with, then you're talking hours and hours of fun. Not just that, but there's lessons to be learned (like what floats and what sinks, how to skip rocks, and many other interesting things), and playing with the water provides all kinds of opportunities for imagination. When Max wasn't crossing the ocean in his amphibious monster truck (which floats thanks to the large tires), he was diving for buried treasure (provided by his brother who had sunk every toy that didn't float). He even got out his Max sized shovel and helped me build up the dirt on the backside where it was a little low.
Another advantage water and dirt has over other kinds of toys is that there really isn't any shortage of it or anything special about one bit of dirt over another. It gets to be rather a pain sometimes when we'll get some other toy for one, and within two minutes the two of them are fighting over it. Tomorrow that same toy is going to be languishing under the coffee table from inattention, but at this moment its the new thing and they're in a struggle to the death over it. It gets bad enough at times that we've ended up with two of every significant Thomas train, two sets of the major Hot Wheels Acceleracers and many more items, all the way down to matching flashlights. Even that doesn't always help as somehow they can tell the difference between two identical objects and start fighting over which one the other one has. Luckily Oscar is still too little to get in on the action and I've told my wife that I am not buying three of everything.
So the mud is a great equalizer. There's enough dirt for everyone. And after the kids have had enough for one day (and have been thrown in the tub to get clean) even the wildlife gets in on the action...