If you're with the military, do some research and find out what vehicle has the highest price spread between the states and Spain. Buy it, have the army ship it and then sell it while you're there.
Do the same for the return trip.
I know a guy that did that to Australia, and the highest spread vehicle to import over there was, believe it or not, a Pontiac Fiero.
Dont remember what he brought back. I think it was a mad max Falcon XB
Most countries allow just about any car if you're a tourist, or student and the car stays registered in your native country.
If you're not with the military, just buy a cheap car to use while you're there, the shipping costs would be more than the EB is worth.