everything will b fun if you have friends to chill with.
good places to shop/chill/eat - 강남/명동/동대문/신촌/홍대(lots of clubs here and lots of 고딩s/대딩s)/ and most importantly 면세점

cheap
good places to go for watching/sightseeing - 혜화동(대학로) for watching musicals (they are cheap and pretty good)/한강(go to hangang with friends at night with chickens and beers)
good places to go with no reason - nightclubs/clubs (yup)
there are way more places to go but rly, if u have friends to chill with, newhere is fun. would b more fun if u have 이성 친구

and places to eat yummy foods is everywhere so wont b so hard to find.
umm just go enjoy.