I like the sensory deprivation and a bit of "Clockwork Orange" "Ludwig Van".

It goes like this:

You record some long, specific classical or jazz song you use during sessions. You can blindfold, put headphones on the sub and perhaps Vapo-Rub under the nose so they only breath through their mouth. You space out your playing at random times so they have no clue when the strokes land.

Then - when you are in public or coming home and you are in the "wait till I get you home.." mode, you pop in the CD with the punishment song and watch the association make them squirm.

I like the water frozen into a condom. Can you freeze a handcuff key in side, put the sub in bondage, insert the frozen condom informing them it has to melt so you can retrieve the key?

New Ideas (ok, really an old idea)

The "Chinese Pony":

Get a sturdy kitchen chair and have the sub straddle it backwards. Loosely tie the wrists to the back with enough slack they can grab the back to keep balance. Put a piece of 2x4 wood (sanded) along the seat so the sub has to straddle it. Pull feet backwards and lash to the legs of the chair or to some of the wood sticking over the seat.

Most of the subs's weight is centered on the wood that grows more and more painful over time. They start to rock back and forth which mimics riding a pony. The sub CAN lift themselves up a bit with their thighs and hands - but not for long (and face it - watching them wiggle to relive the pain is part of our fun)

Use a timer and give them correction by adding 5 minutes per infraction.

For more harsh punishment: Use 2 chairs back-to-back with the 2x4 between them. Sub is between chairs with full weight on the wood and hands tied to one chair to keep balance. Feet pulled up and lashed to wood.

The cool part about this: simple items from home.