There is plenty to see and do at one of Seoul's tallest skyscrapers.
A trip to the observation deck is well worth the entrance fee, but make to go when the skies are clear. On a good day you can see out to Incheon airport.
There is an aquarium in the basement, the penguins are the popular and plenty of fish to see.
[However you will probably see more fish at the Noryangjin fish market just a few minutes drive away ]
The Food court is not the best, there aren't that many choices and it was mostly fast food style, but the buffet is supposed to be one of the best in town. It's open for lunch then re-opens in the evening for dinner.
There is also an IMAX cinema.