My first thought was to have a baby carrier left on top of the car, zooming down the highway at 80 mph, but thought better of it...somebody (me) might get killed!
Step 1: Powerful Magnets
Harbor Freight has these "90 lb. Pull" magnets for like, $5.99. I think the force is exaggerated, but the price is right, so I buy two at a time and click them together. I figure the second one adds 50% more power, but also adds more weight.
Step 2: Prepare Object
You will need to select your 'cup' carefully. I used to use the "Big Gulp" cups you could find at gas stations, but they don't have them anymore, so I went to Dollar General and bought this bowl. The bottom needs to be flat and thin, to maximize the magnetic power. The plastic prevents the magnet from scratching the top of the car, but it needs to be as thin as possible. A shopping bag would be ideal (IN FACT, WHY NOT LEAVE YOUR SHOPPING BAG ON TOP OF THE CAR? STUFF IT WITH NEWSPAPER AND TIE IT CLOSED!). The bowl shown in the photo catches a lot of air but I've been driving around town at 45 mph, making tight turns, no problem. People are honking and waving and yelling, "HEY MISTER, YOU LEFT YOUR THING ON TOP OF YOUR CAR!" and I yell back, "April Fool's!". HaHaHahahahaha ha ha HAAAA! (manically evil laugh)