Traditionally, the groomsmen are in charge of decorating the wedding getaway car. The wedding getaway car should be hidden away and premiered only when the bride and groom are ready to make their final exit.
Some couples opt for a wedding limousine. While a limo is a classy way to exit the celebration, it is almost always more expensive, and there is usually not as much freedom to decorate. Spray paint, for example, might be out of the question. Whoever is in charge of decorating the car must make sure to speak to the limo company beforehand to find out any regulations or restrictions the company has (the same goes for any type of wedding car rental). Tying old shoes and tin cans to the bumper — or tying streamers to the antenna and door handles — is probably harmless, but it's still important to verify. Be careful if you choose to decorate the wedding getaway car with balloons, as there is the possibility of obstructing the driver’s view. Also, you need to make sure that the limo driver arrives at the reception early to allow for enough time to decorate.
Another idea to consider is using a static-cling decal, which is a sign that attaches to the rear window using static-cling (so it can be peeled off easily); it appears as if the sign has been painted on. This also makes for a great wedding keepsake once the festivities are over.