Improvisation here, but in my observation the turning bar remains one of the most popular (and simple) playground pieces. They feature in almost all of Aldo Van Eyck's work. I don't see them much in newer installations, though...liability, perhaps?
ByChusseau-FlaviensTitle, from the George Eastman House Collection. Online at the flickr commons project.
I keep thinking about this turning bar...I really like the length of this one because it allows for communal play in a way that the typical short length of bar does not.