What's one place that's really nice but tourists never know about?
I don’t know exactly what you mean by place and nice is definitely relative but one of my favorite ~unknown to tourist~ areas in Seoul is Noryangjin (노량진). The Noryangjin fish market is pretty well known but I don’t mean the fish market, I mean the actual neighborhood near the station (Noryangjin station line nine exit 3)! It is always really high energy and full of young people and has tons of cute cafes and restaurants and drinking places. The best part about it is it’s known for having the most hagwons in Korea so everything is CHEAP AF. The resturants and cafes and drinking places are all so cheap to accommodate the university students that are attending the hagwons so ITS GREAT. I’ve found so many great delicious places to eat there and really nice cafes that are fantastic for studying or if you just want a quiet place to relax. There are also marts there that sell foreign snacks and such which are also really, really cheap as well as a entire huge street just lined back to back with really amazing (and again - cheap!) street food. I’ve never seen another foreigner there in the countless times I’ve been there and I’ve never seen it mentioned in any ‘places to go in seoul’ sites so that is my recommendation. Its not fancy or modern or asthetically pleasing but it has a really great atmosphere and is somewhere no one ever really talks about.
If you want to know more places, I have tons that I know of in Seoul and outside of Seoul I just need more specific criteria! And if the place I mentioned isn’t what you were looking for, I’m sorry! But I do think everyone should go to Noryangjin at least once it’s pretty cool.