If you want to decorate your getaway car, the Classic Bride offers some great ideas using real flowers as well as ribbons. My personal favorite is the light pink VW bug.
If you're renting a getaway car ask the company about some alternatives to a traditional white or black limo. Some may have a classic Rolls Royce and others may have vintage cars in their inventory. Or if you only want a vintage car search the Internet for "vintage car rentals" in your area.
Once you've selected a car, find out what you can and can't decorate it with. The company may recommend a paint or adhesive that can be removed without damaging the car. Or they may not want you to paint "Just Married" on the window but tying ribbons or the familiar cans and shoes are okay. You don't want to do something that can damage the car and get stuck with a bill for repairs.
Also ask about when the car needs to be picked up and returned, and if you're leaving for your honeymoon early the next morning ask a friend or relative to return the car for you - and make sure they know where the keys are!
If you're making your getaway during the day all of your guests will get to see the car and it will make a great photo op, but if you're leaving the wedding in the dark it may not be worth the money to spend a lot on a car and decor no one will see. Since we were the last to leave the reception - with a few relatives - we drove to the hotel in our own car sans decorations.