You could go for something romantic, and sweet....will you be spending christmas eve together? Maybe buy her a pair of crystal wine glasses and then when she unwraps them produce a bottle of nice wine and sit around watching It's a Wonderful Life or something of the sort.

I'd skip on jewelry, lingerie, or flowers and candy, that's more valentine's day stuff. If there's enough time before christmas you could get her an ornament, one of those engraved ones with the date and your names. But not unless you're pretty serious. I assume she knows you like cars? How about a return to Tiffany's keychain? A little pricy but you don't get a better reaction than something from Tiffany's...it shows it's from you because it's car-related, but shows you care about her because it's kinda like jewelry...