Really, don't worry about the accommodations. I don't really believe in Guest Houses because i've heard that some guest houses are really run down as they have quite high tourist occupancy..
Try any local motels that you can find on the streets.. The cheapest i've found is 30,000won which was nearby the local university (which allowed three people in the room thank goodness) and the most expensive is near MyeongDong, with prices ranging from 45,000won - 80,000won.. Don't worry about not having any place to stay in Korea!
Plus, if you stay in Motels, they will usually kick you + your luggage out by noon.. And it will be quite expensive to check in Before 7-8pm..So usually if I go travelling in Seoul, i will carry super light luggage because i will be lugging it about the entire day ..
Oh yeah.. but the motels are abit seedy looking lol. After all they are there for people to.. heh for a few hours and then leave.. If you don't mind lah, its the cheapest form of accommodation (in my opinion lol) And you need minimal korean language.. like to ask how much is the room, and asking to see the room first before paying and stuff...
Lotte World and Everland admission tickets would cost probably RM150-200.. I can't remember, print discount coupons off their website

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And yeah i agree with Ycl90.. lets say you don't have breakfast, have super light lunch and splurge for dinner! When i went to Everland & Caribbean Bay, we didn't eat breakfast and lunch and had fantastic dinner.. So.. its up to you lah~
I have been to the aquarium in Busan.. frankly its so so lah .. i mean on a scale of 1-10 its probably like..6 or 7.. but do come down to Busan.. different kind of environment. Sorry a little biased lah but i prefer Busan than Seoul

less people and beautiful place too, Busan

However if you want to come down to busan, the cheapest train (and the slowest too lol) would cost you around 25-30,000won (and that is extra money that you have to spend lol)
Oh one more place i should recommend you to visit is Sorak Mountain.. nearby Seoul (I haven't been but my friends say its super beautiful)~
And Itaewon is a lie ! Lol. I went there last month and .. . . . its just a street full of foreigners cheating other foreigners. I didn't walk around much but was really quite disappointed with Itaewon.. T____T
Sorry my response is so unorganised.
Has anyone been to Gwangju or Daegu? Thinking of going there before i leave ~