Well the kids just love to "race" around the yard with their trucks and bikes, and anything else they can find, so I made them a Race track. This is quick, cheap and easy. It took me 10 minutes tops and a can of spray paint. Use your imagination and paint a track on the grass in the back yard. When the grass grows, you cut it and its gone. My only suggestion is don't make it so small they get bored and don't make it so long they give up on the finish line before they make one lap around. This kept my kids and the kids I watch busy for 2 days! You can make it as simple or complex as you want. I set the little picnic table up next to the track with a pitcher of water and cups and the kids dubbed it their "pit stop".
You can see most of the track in this picture. It just loops behind the pool and comes right back the the start line in front of the swings. They love it when it goes under things(the slide) and around things.
Garrett and Josiah at the Start line.