I am just back from 5 nights in Seoul as part of a wider Asia trip. This was my first visit to Seoul.
Before hand I had watched plenty of YouTube videos but not really settled on what I wanted to see or do and ended up winging it day to day - this was a bit of a mistake, in that I should have spent a bit more time making a list of places I wanted to see and also researching where they were in relation to my hotel in Hongdae.
I found it similar to my first visit to Tokyo 10 years ago where I feel I only scratched the surface of what there was to see and do.