One of my favorites is the remote-controlled vibrator:
http://www.bettersex.com/vibrators/s...rator-333.aspx
She wears it, and I have the remote. I love to watch a woman try to stay composed when I hit the button just as a server comes up to take her order. The trick is to stay cool and casual and to tease and excite, not to spark a "When Harry Met Sally" restaurant scene. The fact you have to stay composed and act like nothing's going on just adds to the buildup.
One of my ATFs (since retired and relocated) used to travel out of town with me on business and would always wear this when we traveled. She once fell asleep in the car and I used it to wake her up; it makes for a really fun alarm. Another time I used it intermittently in a restaurant; she never knew when it was coming. She was so worked up and right on the edge as we were leaving that as soon as we got in the car (which I had deliberately parked as far away from other cars as possible), I hit the button so she could finally finish. She was shaking and screaming in the car as I pulled on to the street. It was incredibly hot.