Yeah thing with a Hummer is, they're a commonly available US domestic car, and it wouldnt exactly leave a lasting impression on him.
I wouldnt like to go to USA and be driven around in a commodore
Id lean towards ISF, Lambo, Ferrari 360CS/430, Merc black series CLK, Porsche GT3 or GT40. Maybe a nice Aston, but i think Universal wouldnt have that much taste in cars for something like this, as its more of a "wow" than a "oooooh"
Although, seeing how many passengers there are, these spring to mind; RS4, Masarati x 2, Bentley, Ferrari 612, GTR Q7, and yeah, the H2
But yes James, dont leave us in suspense