We visited there a couple of years ago because my GF's daughter was teaching there. It was a great trip. We stayed in Seoul (with the advantage her daughter being able to give us a "lay of the land"). The subways are easy to use with an ap. that works great and there are a lot of historical sites - i. e. temples to see.
Street food is plentiful around the city, including in market areas and very tasty and safe to eat.
We went to the DMZ (on a tour) and because of all the history and news today, it is well worth it. And yes, you can go into the blue buildings where you saw when the 2 leaders meet there recently and hug. I don't think Kim is still available for hugs there..
My GF flew to Jeju Island for a couple of days another trip and said it was great.
We took a train from Seoul and spent a day at the beach in Buson. I still have my sunglasses that I bought from a street vendor. Street food is once again plentiful and good.
For women that like make-up, it is a huge business in Seoul.
Enjoy.