Balancing Obstacles
Balance beams are a fun part of any obstacle course. Balance beams are easy enough to make with a few stacked bricks and a long board. For younger kids, keep the balance beam relatively low, of course--and make sure to test the beam for stability before letting children run over and around it.
Jumping and Crawling Obstacles
Obstacles that force children to jump around are a popular part of obstacle courses. For more permanent obstacle courses, get a couple of old tires and place them in a zigzag pattern for kids to jump in. For an obstacle course that you're only going to set up for a little while, this can be done with hula hoops as well.
If there's a paved area, you can draw hopscotch squares to serve as part of the obstacle course as well.
Weaving and Steering Obstacles
You can use traffic cones or sticks to create a line of obstacles that children have to zigzag around. It's okay to let them run around the traffic cones, but you can add an extra challenge by having them ride a tricycle or a scooter around the obstacles or having them run or walk backwards.
Water Obstacles
Particularly on a hot day, a water obstacle might form the piece de resistance of your obstacle course. You can make a slip-and-slide if you have a hill, or set up a sprinkler at the end as a course finale. You could also set up a wading pool for children to run through.