SOME PEOPLE RETURN from a trip with a stash of mini bath gels in their carry-on. The more daring might nick a bathrobe. But who's ever brought back a shiny new Lamborghini—and with the hotel's blessings? Properties have been upping the amenity ante with new car-related perks, aimed to make sure you sightsee in style (and, in one case, to send you home with one hell of a souvenir).