Those are good suggestions.
If you like seafood, you might consider visiting
Noryangjin Fish Market. It's been renovated, and I confess, I only visited prior to the renovation (last time in 2015), but it's very interesting for a few hours. Get some food while you're there.
If you want to get out and about, back in the day I went out to see
Freedom Park in Incheon, which isn't far from Wolmido, a location which figured prominently in the landings in 1950. IME getting out and about is good, and you don't need a major attraction to consider a particular destination. Joy is in the journey more than in the destination,
this park has some backstory.
Trip Advisor has a long list of possibilities; you might get some inspiration from that, or
from Life In Korea.
If you're looking for something more active, but don't want to cycle, you can hike. There are many places all around Seoul. I've hiked to
Dae Nam Mun a couple of times, once in the winter and once in the fall,
which is on the outskirts of the northern part of Seoul. It's just an example, but you can get out and see the fall colors.
Finally,
you might consider hiring a local.