More ideas from Cas
Natural forts: a grape arbor, a bean tent, etc.  Read more about Natural Playgrounds on Quiet Nature: http://www.quietnature.ca/natural-playgrounds-are-on-the-rise-in-ontario/

Make Room for a to hide are often a neglected part of the playscape because of parental fears. Just remember that your goal is partial, not total, concealment. There are many options, but be sure to see the den as a less perman