DomBob I like the the frozen key Idea.
One I've been toying with lately, but have not had the chance to try out, is simple but every sub I've talk to about it has hated the idea. Bind their hands behind their back and have them kneel on what ever pain inducers you like, uncooked rice, pencils, frozen peas whatever. Then you set a bowl full of ice water on there head and have them balance it. Set a time limit for the punishment and every time they drop the bowl time starts over. To make it extra challenging use a glass.