I like the graphics idea, but you would definitely want her input. Maybe talk to the shop, get some ideas and give her a gift cert? She might like flowers or some theme shes partial to - say a movie, a band, a hobby (gardening, travel?) If you present her with ideas of things you already know she'll love it'll be really special.
I am partial to graphics, as mine has a racing stripe and I LOVE it. On the other hand, some people would rather blend in to the crowd - but those people don't usually own/love smarts.
How about a professionally done photo of her car? or her with her car? Sounds silly, but if she loves her smart, she'll love pics of it.
I'm hoping my sons get me an i-pod (yeah, I don't have one) and they need to load it up with all my favorite music. That'll be so cool! (Kids, are you reading this?)
Good luck and let us know what you decide.