N Seoul Tower
I did all sorts of touristy things during my first trip to Seoul, and one of the places I visited was N Seoul Tower. I went there during daytime, since it is more popular at night.
There are different ways to go to N Seoul Tower. You can go there by cable car, by bus and even on foot. I chose to go by bus and walk all the way up to the top. I then took the cable car to go down, which I believe was not the best choice.
Aside from the majestic view, N Seoul Tower is popular for its colors at night, and the rows upon rows of love locks.
TIP: Bring your own padlock if you want to leave your behind. Padlocks are a bit expensive if you buy them on site.