You can take a bus ride or a cable car to go to the top of the Namsan Tower. Cable car hardly cost 6 USD for one side per person. You need to buy a separate ticket to go to the top of N-Seoul tower observatory. You can see the Skyline of the Seoul with a breathtaking view.
You can also do shopping for candies and souvenir at the observatory deck. The view of Seoul city is amazing at night too, If you get too busy in the day time and miss the chance in the daylight then you can enjoy the romantic dinner with a lovely view.
There is one more souvenir shop in the basement of the tower, where I picked few fridge magnets to add to my travel collections :) There is also a 7-eleven kind of store to buy basic items.
if you like to make a wish for your love on, you can buy a love lock at the shop and lock it along with the other locks.
Overall this place is a must visit for 3-4 hours of duration with hardly 10-15 USD of expense.
I hope you enjoy this place when you visit to Seoul.