Introduce a $5 cash incentive for anyone returning trays.
Station a guy with a bag of money at the tray station to hand it out.
Singaporeans will be competing with each other for the trays to return.
Soon foreign workers will get into the act, People will be hovering above you as you sit eating your meal, waiting to snatch away your plate to bring to the tray return station and claim their reward.
Uncle Fai's relative will now insist on returning all the trays, bowls and plates saying that it is the compassionate thing to do, since the cleaning aunties and uncles have been overworked already.