You get a card posted through the post. No card no park?
Land registry and post office are involved. Spot checks.
Policed by the people who usually run the parks cafes, pedaloes and crazy golf ammentites
Maybe another idea is a wrist band idea like the leisure centre so green orange blue pink and tannoys say when you're time is up and you have to start heading home
Disclaimer: Sorry only just seen the thread and not really thought about it. And these are only half thought ideas