Christmas in Korea is not as 'festive' as it is back in the U.S./Canada
but there are still things to do/see in Korea (palaces, Korean plays/musical, museums, etc)
shopping in Korea could be cheap or expensive
imported stuff are expensive ... expect to pay a lot more here, BUT Korean are pretty 'trendy' and things go out of style quickly and you'll see them at a clearance sale sooner than later
there are tons of shopping places (clothes) in Korea as well
but do your research ahead of time or get a guide, English is not spoken well here
another good thing about Korea/Seoul is the transportation/subway system, you can get anywhere in the city easily with the subway