Some ideas from an article on the web:
Let me preface this by saying that you should never expose the public to any kink, fetish, or even Vanilla sexual play. Only do these punishments if you’re certain you can be discreet.
Long-Distance Sex Toys. There are lots of Bluetooth and internet controlled long-distance sex toys available today. Some are small and discrete and can be worn in public. Put one on your sub and make them wear it on a date. Control the intensity with your phone. If your sub makes even a tiny noise, the punishment gets worse at home – one idea is to leave them without orgasm after hours of toy-play. Or put a toy on your sub, stay home, and make them do errands. Reward them by finally bringing them to the climax when they come home.
No Clothes In Public. Send your sub on an errand wearing just a long coat, or long coat and pants. This punishment doubles as a productive activity, so it’s extra impactful if it’s an errand for you, not one of their own.
Ben-Wa Balls. Ben-Wa balls aren’t as much fun for doms as are controllable sex toys, so they’re not great for the date-night scenario. But making your spouse wear them during an errand can be uncomfortable and embarrassing.For the untrained user, they can be hard to hold in. For male subs, use a weighted butt plug with too much lube instead.
Silly Costume. Make your sub wear something silly in public. Have them wear a costume from a movie, paint their face like a cat or wear ridiculous, knee-high rainbow socks. Whatever you want! Don’t go overboard with this – you don’t want to humiliate them. You want to make them look slightly unusual. You can put them in humiliating clothes… but that’s a thought for a different category.