In this case we suggest magnetic signs/stickers. This is because the chances of the paint getting damaged/scratched are much lower, you can easily remove the magnetic signs. However, depending on the size of the sticker, it can be difficult to make it stick for long periods of time.
Note: Leaving the magnetic sign on the car for more than a couple of days may lead to a difference in colour between the spot where the sticker was placed and the rest of the car.