One child roundabout
I was visiting my mother in Kenilworth last week and we went for a walk in the nearby park. I used to take my children there when they were small, the most interesting piece of apparatus was a one child roundabout which I was amazed to see is still there. The idea is that the child hangs onto the central axis with a foot on the platform at the bottom and gets it spinning. Once spinning they pull themselves into the centre, reducing their moment of inertia which results in an increase in angular velocity. I of course explained this to my children but I'm not sure they remembered any of it at the time. The thing I most like about it is the way the axis is bent so that the children can get their bodies as close to the centre as possible. It works particularly well if dad helps to achieve a decent initial angular velocity.
Play grounds have some great possibilities for student investigations and EEs.