I found this playground toy. When I sat on it and rocked backwards and forwards, the time period was about 3 seconds.
With no one on, the time period was less than a second.
I'd hoped that perhaps it could be modelled as an inverted pendulum but clearly not. There will be an elastic restoring force when it is displaced. I'd have to go back to my degree work to remember how to calculate that and thus figure out if the motion can approximate to simple harmonic motion.
