I have a one mile cardio trail at my house that I use to train my endurance horses and me, when I run. It would make a newbie and even some ACTHA seasoned competitors cringe! But it will give you ideas - up and down steep ravines (switchbacks), figure 8 BACKWARDS around two trees, water crossings, jump narrow ravines, reach down and drag huge limbs out of the way without dismounting - forwards, and dragging backwards - walk backwards UP a hill and stop, then pivot 180 to go the other direction (while still on a slope). Sidepass the length of a small fallen tree. Back in the shape of an "L" without veering off course. Walk down, thru and up ravine banks in a calm, quiet and efficient manner (pushing with the rear, not pulling with the front legs).... can you tell I prefer the wild woods to the arena with poles and manmade obstacles? LOL! Oh and then let's not forget the occasional wildlife...sometimes they can be an obstacle, too!
Watch some ACTHA obstacles on Youtube - great ideas. Some are really educational and some are just too darn funny!