Uhh. Probably not.
Lotte World is indeed childish, but there's a more "grown-up" area outside. This is where you'll spend 2 hours in line for each ride. Still some decent rides though. Go early, leave early. Prepare to be touched repeatedly while standing in line.
I think you'll have the most fun in myung-dong if you go at night. That's when its craziest.
Cheongecheon is LOONG so you can probably squeeze that into some of the other parts you want to visit; plan ahead. It's also prettiest at night, but it'll also be busier.
Coex itself isn't really worth visiting, unless you just want to walk around. There are some interesting shops however, especially if you're a tourist. Aquarium isn't terrible either.
The flea market, I would probably cross out. It's not as interesting as they make it sound, and it's pretty far out, so I think you'll be spending more time than worth.
Always account for extra travel time, because the subway stations are huge and it takes forever to leave them. Morning/evening rush hours, prepare to get touched by many people simultaneously.
Have fun though
