Lol. Just kidding. I mean, the stuff they said is true but THERE IS MORE!!
I live in the Sinchon, Hongdae area and this is where you come for samgyeupsal, chicken, and SHOPPING. Clothes, trinkets, makeup - its all here. The nightlife is pretty legit here, too. Karaoke, live performances, bars.
Korea also has a thing called meat bars. Yes. We are so obsessed with meat that we have buffets solely dedicated to them. It's mostly pork though (but there are places in Sinchon where u can galbi for cheap). There is one in Konkuk which also happens to be a 10 min walk from Super Junior Mouse & Rabbit Cafe (where they serve THE BEST frappuccinos. Green tea is my fave).
And speaking of bars, if u like alcohol, we also have infinit3 refill bara. Pay one price and drink as MUCH as u want. Icon in Hongdae is popular. B-lab is popular franchise as well. If you're not much of a drinker, don't bother but if u like cocktails or just wanna try every different flavor while you can, then DO IT.
Now GANGNAM. This is where it can get tricky. I'm sure you know about the area everyone talks about - the one right outside Gangnam statiom exit 11 with its underground mall and foreign-popular streets. Yada yada yada.
I, on the other hand, barely venture there. If you keep coming down further, you'll get to the REAL hidden gems of Gangnam: nonhyeon and sinsa. Nonhyeon may or may not be the red light district but this is where u go for cheap eats and THE BESTEST MOST SPLENDIFEROUS SMOOTHIE PLACE EVER (85 st kbye). Sinsa, however, is the real winner. Garusogil is THE place to go for everything: shopping, eating, drinking, clubbing. Cheongdam and Apgujeong, too, but these places are strictly Look But Don't Touch. This is where the ritzy live BUT its also where the entertainment agencies if you into that. And pssst: this area is where the famous hang out. Who knows? You might get lucky. Or at least you'll certainly see their autographs everywhere. Play "where's my bias?" Hahahà

Everything i have read in the below comments are true. That is my contribution. Good night.

I know that was aALOT of information....but YOU DID ASK!!

If you get there in the warm season. go to Banpo Hangeng and see the light and water show off of the bridge. Beautiful!
If you plan to shop, do what I did. My packed luggage was inside a larger, empty suitcase, then I had essentials in my carry on....Everything was STUFFED when I came back!
Now, when I planned my trip, I knew some of the things i wanted to do. So I looked up locations and subway routes. Once you know where you are staying, do that and minimize wasting time lost in Seoul...although I DID find some AMAZING places by just wandering. I looked down an alley between a trophy shop and a convenience store and found a gorgeous old Buddhist Temple! Online, you can look up an english labelled subway map..VERY handy!! If you can print in color, do it and take it with you.
Good shoes. Be SURE you have comfortable shoes. Seoul is built up and down HILLS..Gangnam is hilly...you will KILL yourself in heels or new shoes....and take a compact umbrella! It can rain at the drop of a hat and is useful for when it is hot and sunny out too!

Now through the fall is the warm season in Korea. If you go after October, be prepared for ANYTHING!! It gets MIGHTY cold there...in the negative numbers at night. ALso, depending on where you are travelling from, check to see if you need any immunizations. I went in the spring and it was recommended to take anti Malaria drugs for two weeks before, during and after my stay. IF you do stay at Dolce Bita, be aware that they DO NOT provide food! Stock up before you get there. If you arrange a pick up and ask, they will drop by the local supermarket with you.
Soju...one word says it all. you can get it cheap at the corner stores. TRY it at least once. They also have citron and peach flavors. Food...be prepared to eat yourself silly! The food is OUT of this WORLD! Spicy is the name of the game, but you CAN get not so spicy varieties. There IS western food! Kentucky FRIED CHICKEN, Dunkin Donuts, MCDonalds, Burger king, Pizza Hut..they are there, but sometimes hard to find. Red cap pizza is a local chain that is good. Also, Lotteria.. they have good food as well, but by all means try the local fare. I advise you to learn at least a few polite phrases. Thank you, excuse me, please. Polite is the name of the game. I learned a few phrases as well as terms of address and was far better received than the people who just looked blank faced.

http://www.dolcebita.co.kr/main.html
I f you want to stay at GD's pension, you can book online here. It is about an hour outside Seoul in Ildong, but you can take a bus and call them for a pick up.
AND.here cafe..Seungri's place...
90 Dongsung-gil, Jongno-gu, Seoul, South Korea +82-2-744-8464 http://www.andhere.co.kr You can leave a gift at the fan wall.
Now. before you leave the airport. visit the Mpass kiosk and get a transit pass for a week. it ran me about 80 USD for a week and the pass is good for 20 swipes a day on the subways, blue, green and yellow buses. The pass is MUCH easier than trying to get tickets at the subways or paying on the bus.. cheaper too!! And when it runs out, just take a subway back to the airport and renew it for a few more days! You can either rent a cell phone or get a sim card for Korea at the airport too. much cheaper than elsewhere. DO NOT MISS the palaces. Even if you only go to one of the five in Seoul, its worth it. The wall walk is another great thing. You can walk a section of the ancient wall of the original city of Seoul. Its incredible to see that on one side and the modern city on the other...
SM/COEX is a MUST! Maybe you'll get luck like I did and see an idol or two! I met Jongin and Baekhyun in an elevator there. YG is harder to locate, I suggest a taxi.

If you're going in winter, prepare for very very very cold weather. I used to live in Korea for about 9 years, and during winter it snows a LOT! Pack warm cloths.