Ok, well be prepared to be spoilt off your feet!
Koreans are very hospitable, and will spend gazillions of dollars to make sure you are having the most fantastic time.
Try to give something that really reflects america's culture or tradition.
Perhaps a nice hard cover book with photos of some of Americas greatest natural sights. Koreans love nature (althoug they dont do much to protect it), and tramping.
They wont expect much from you, but if you do give something it should have some careful thought to it.
Not too sure how old you are, but a nice bottle of american wine would be nice.
Its going to sound harsh, but Koreans think that everything made in Korea is better anyway... but my korean friend thinks either of those two would be good.
Have fun! You're going to love it, especially being hosted. I remember those days in Korea.