It's really hard to write a long post.
The reason I wrote this much is to promote Busan haha.
Busan has many elements compressed into it, including urban skyscrapers, the sea, mountains, rivers, and rural villages. So, even if you walk just a little bit, the scenery completely changes.
You can get to most places by public transportation, and Busan seems to be the only place with well-planned trekking courses between photo shoot locations.