Seoul is bright and busy most hours of the day, evening and early dawn. Koreans do not spend a great deal of time in one locale; instead they move from place to place. Your group may start playing billiards, then off to dinner, then singing, then to a cafe, then to a nightclub, and who knows where else? Each of the following neighborhoods have a bit of the above activities: SHINCHON/E-DAE (near Yonsei and Ewha Women's Universities); MYUNGDONG, CHONG-NO, INSADONG (Central Seoul); DAEHAKNO (University Street, Hyehwa Subway Stop); KANGNAM (Southern Seoul, including Apkujong-dong--this area is more expensive than others); and I'm sure I'm missing other parts of the city. Also, there are 2 areas in Seoul, Mia-dong and Chongnyangni, where prositution & brothels plentifully exist. I hear the girls get younger and younger. Korean citizens can't gamble, so explains underground gamling. Non-Koreans can go to Sheraton Walker Hill Hotel (and probably other establishments) and play black jack or the slot machines. If you do choose to go, remember to bring your passport.
Dress Code: Certain nightclubs have a dresscode. These clubs tend to have a 'minimum' order per person and can be selective as to who can or can't enter. Other clubs (e.g. 'rock cafe' - a casual dance club) have no code. Ask before going.
Once Upon a Blue Moon has live jazz nightly and makes a great date spot in an intimate, candlelit location. Drinks are a little pricey but worth it. OIBM is also much dressier than most, so don't wear your blue jeans and sneakers!
Black Bush Whiskey named it one of the World's Best Bars... (http://www.worldsbestbars.com/city/Seoul/OnceinaBlueMoon.asp
Dress Code: slacks, dress shoes, nice shirt minimum. Some will have coats and ties.
J's is located in the alley beside the Hamilton Hotel and KFC in the heart of Itaewon. It is large, but quiet and dark. They have live music several nights a week. On weekends, they have several bands perform for about an hour or two each. Nice place to go with a group to enjoy music, but quiet enough that you can go with your significant other to sit in the back and talk.
We never spent much time here. Some nights it was so quiet it was almost spooky; other nights it was too busy to be much fun. I'm always looking for that happy medium!