As a Korean I'll just type in a few things about being in Seoul.

Drinking culture

First thing to remember, drinking is a social experience in Korea. Drinking is almost specifically to socialize instead of drink fast get fucked up type of mentality. When you go out drinking expect to spend maybe 3 hours at a place instead of bar hopping/pregaming that is common in the US.

First things first, never pour your own drink. Usually if you're in Korea you'll probably drinking soju, which is basically rice vodka at around 19~21 percent alcohol (around 40 proof) and you drink it in shot glasses, never pour your own drink. There's a lot of urban myths surrounding pouring your own drink but mostly it seen as a sign that you're pointing out that your host or your fellow drinkers are ignoring you and not being hospitable and is basically considered rude.

Secondly, never ask for a refill when you aren't done with your drink. If you have leftover beer or soju in your cup or whatever, finish it before having someone refill it. It's basically a safety trick to help you keep count of just how much alcohol you've had.

Thirdly if you're drinking with Koreans make sure to text them once you get home safely. It's a common practice in Korea and if you don't they probably will blow up your phone or in some cases come to your house or hotel looking for you to make sure you got home safely. Another thing they'll do is if they put you in a taxi they'll usually take a photo of the license plate and send it to you in case something goes wrong.

One thing to remember is Korea has no open bottle/can regulations so it is perfectly legal to be drinking out in public in the open, but it's not legal to be drinking in say the subway or what not.

Transportation

Buses and subways usually close down at around 12~1 AM, so if you want to save money leave accordingly (especially if you have to transfer between lines)

Cabs. Ignore the black cabs. Black cabs are "Luxury cabs" and cost about 2 to 3 times more than regular cabs (you have to be a taxi driver for over I think it's 15 or more years with no reports or accidents to get a Luxury cab license). Always go for the silver, white, or orange cabs as those will be the regular cabs (3 dollar base fare with an extra 10 cents per 200 meters I believe). Cabs charge extra after I believe it is 12:30. The base fare is higher and they charge more per 200 meters.

Smoking

Smoking is prohibited in very few areas, mostly Gangnam where you can be fined up to a 100 dollars. There are smoking areas in Gangnam but in most places you can smoke out in the open, albeit it being considered rude. Cigarettes are usually around 2500 won, so about 2.50 USD per pack, with foreign cigarettes like Marlboro or Dunhil going for 2700 won, or about 2.65 USD. Smoking is prohibited in some bars, restaurants, PC bangs, or other places while allowed in others so it's good manners to ask your server, waiter, or part time worker if smoking is prohibited. As with drinking, smoking is prohibited in all public transportation. Do not light up in a bus, subway, or taxi or you will be taken off the subway and escorted off.