The quickest way is probably to buy something from the Mileage Mall. Since shipping overseas may not be practical, you might want to ship to friends or family living in the U.S.... or send them a nice gift.
You could also rent a car for a day - I think even foreign rentals provide mileage, though you should check with the rental agency.
However, remember that redemptions also keep your account active, so you can also consider doing that. For example, you could donate some miles (I think the minimum is 1000).