My kids favorite spot is our digging spot. I took a corner of the yard, under a big tree and dug out all the grass. I have real hand shovels as opposed to the cheapy sand toys. The shovels I found at Wal-Mart are sturdy plastic.
I rotate out tractors, dirt movers, plates and pie tins, hammers and golf tees to pound in the dirt. (My favorite part is that instead of telling kids "No! Don't dig in my yard!" I get to remind them that there is a spot that they are allowed to dig in.
For the wheels, we put in a cement slab with a loop of a sidewalk coming off of on one end of it. It makes a great track for tricycles and coupes. Plus, I have a couple of scooters for the school age kids. The slab has an adjustable basketball goal and a little tykes goal.