Tires
Old tires have many potential uses in an obstacle course. Set them in two rows flat on the ground, side by side. Put the tires in one row slightly behind or ahead of those in the row next to it. Use this setup as a track for the children to run through, stepping only inside the tires in the right row with their right feet, and only in those in the left row with their left feet. Hang a tire from a tall tree or pole and have the children jump up to grab it. Alternatively, have them throw balls or beanbags through the hanging tire. Lay the tires on the ground in a zig-zag pattern and have the children run the zig-zag for the classic weaving drill. You can even use them as weights for the children to carry. You can find cheap tires at tire retreading stores. Check them for worn patches where the belts are poking through, or sharp objects buried in the rubber, before using them. (See References 2)
Boards
Lengths of wood, such as 2x4s, also offer a wide variety of options for a children's obstacle course. Lay one on the ground for a balance beam. It requires just as much balance to walk it as a gymnast's balance beam, but because it is lower to the ground, it is safer for the children. Cut the boards to different lengths and lay them down to make a maze. You can screw or nail them together for a permanent maze or just lay them next to each other, so that you can keep rebuilding it and making it new. You can sink a board into the ground like a pole and paint a target at the top for children to hit with a ball or beanbag.
Hula Hoops
The common hula hoop also offers a variety of obstacles. Bury one end in the sand of a sandbox to make an obstacle the children must climb through. Lay them down near each other and make the children jump from one to the next, without touching the ground in between. You can also lay them out in two rows for stepping drills, as you did with the tires.
Boxes
Use boxes to make a maze. You can tape them together for a permanent maze or leave them unattached for a maze you can rebuild. Cut holes in the bottoms of large appliance boxes to make obstacles for children to crawl through.