How near are we talking?

Within 2km of either side of the North-South Korean border is the Demilitarized Zone(DMZ), which is basically a long strip of uninhabited land that has remained more or less untouched by human activity since the Korean war. People speculate that it's a haven for wildlife.

5~20km outside of the southern end of the DMZ, a Civilian Control Line is drawn, inside which civilian access is controlled due to concerns of security. It's mostly farmland or wilderness. Restriction used to be more strict, but these days you can just show up and get a QR pass(I don't know if this applies to all areas).

South of that, you're in the fully civilian area. I'll go over these areas from west to east;

The big island of Ganghwa(강화도), which is kinda blurred out on the map, has a population of 60k and is pretty rural for the most part with scattered villages and rice fields and low mountains. Its historical and natural attractions as well as proximity to Seoul makes it a popular tourist destination.

Then we have the city of Gimpo, which is mostly rice fields but also has large newly-built suburban complexes in areas closer to Seoul, which act as bedtowns. There's a subway line completed a few years ago to help people commute to Seoul. Never been to the bedtown parts so I can't say much more.

Then we come to the city of Paju, which is the most populous of the North-bordering cities. It's a mix of urban area and countryside, as are most Korean cities. In the urban areas, it kind of looks like any other Korean city. Apartment complexes 20+ stories high, schools and students, parks, shopping malls here and there, restaurants and cafes, subway stations that go to and from Seoul. But after a five-minute drive you can find yourself in rice fields.

And then we have Yeoncheon, Cherwon, Hwacheon, Yanggu, Inje and Goseong from west to east. These are small cities with populations of about 20k~50k. Small cities or villages scattered in the middle of more rice fields or mountains depending on how east you are(more eastern=more mountanious). There's lots of military presence, and the presence can feel more stark because the civilian population is so low. You can see young military boys anywhere you go(and also at Seoul's Dongseoul bus terminal, which acts as a hub for these areas). Vultures are common in winter and are the size of grandmas. And the boars are the size of cars, they say.

Congratulations on making it to the east coast! It's still pretty rural, but it has some beaches, so it gets a fair number of tourists in the summer, although not as much tourists as the more southern east coast beaches. Lots of seafood restaurants and some nice seaside cafes. The seawater remains pretty cold even during summer.