If you are going to make him ruin it, make it a humbling experience

Makes sense.

Help keep it foremost in his mind that you own them

What better place for this?

A little cock grab is always good Reminder who is in charge Arrousing Make him have to adjust Incredibly quick and hardly noticeable

Licking his cage through his khakis...  Arousing, so he will love it.  Leaves a wet spot, so he won't want to head into public immediately.

Panties like these...  Fun to show them off to him, and then Make him wear them!  Wearing something that just touched your pussy. Delightful

Burying the keys in the sand = fun. Could bring an extra set and bury a key as the ride comes in.  Men...  Make him wonder which key is buried