When setting up a ninja warrior course without trees, you'll need to get creative. Consider using posts and poles as obstacles or hanging ropes from overhead beams or ceilings. You can also use pipes or tubes of varying sizes and heights to create obstacle walls. For more dynamic elements, look into using balance beams, swinging rings, and ladder swings. No matter what obstacles you choose to include, make sure they are safe and secure. Have a few friends test the course before using it with an audience. This will ensure the safety of everyone involved in the activity. With these tips, anyone can create an epic Ninja Warriors course without trees!