Budget £5. She's very excited about the car and I can't think of anything else 
We're on a friendly joking footing so something taking the piss would be fine, although I have no idea if she has her own car so probably not a bumper sticker in case she's sharing with her mum etc.
She's not really a girly girl, more alternative/quirky/geeky but in a cool way.
Any ideas?